The spinal cord

The spinal cord [medulla spinalis] is the lower (caudal) and most attenuated portion of the central nervous system. It is approximately cylindrical in form and terminates conically. Its average length in the adult is 45 cm. (18 in.) in the male and 42 cm. in the female. It weighs from 26 to 28 grams or about 2 per cent, of the entire cerebro-spinal axis.

After birth it grows more rapidly and for a longer period than the encephalon, increasing in weight more than sevenfold, while the brain increases less than half that amount. Its specific gravity is given as 1.038.

The Line of division between the spinal cord and the medulla oblongata is arbitrary. The outer border of the foramen magnum is commonly given, or, better, a transverse line just below the decussation of the pyramids. Lying in the vertebral canal, the adult cord usually extends to the upper border of the body of the second lumbar vertebra. However, cases may be found among taller individuals in which it extends no farther than the last thoracic vertebra. With increase in stature, its actual length increases, but the extent to which it may descend the vertebral canal decreases. Up to the third month of intra-uterine life it occupies the entire length of the vertebral canal, but owing to the fact that the vertebral column lengthens more rapidly and for a longer period than does the spinal cord, the latter, being attached to the brain above, soon ceases to occupy the entire canal. At birth its average extent is to the body of the third lumbar vertebra.


1. External Morphology of the Spinal Cord

In position in the body, the spinal cord conforms to the curvatures of the canal in which it lies. In addition to the bony wall of the vertebral canal, it is enveloped and protected by its three membranes or meninges, which are continuous with the like membranes of the encephalon: first, the pia mater, which closely invests the cord and sends ingrowths into its substance, contributing to its support; second, the arachnoid, a, loosely constructed, thin membrane, separated from the pia mater by a considerable subarachnoid space ; third, the dura mater, the outermost and thickest of the membranes, separated from the arachnoid by merely a slit-like space, the subdural space.

The intimate association of the central system with all the peripheral organs is attained chiefly through the spinal cord, and this is accomplished by means of thirty-one pairs of spinal nerves, which are attached along its lateral aspects. The nerves of each pair are attached opposite each other at more or less equal intervals along its entire length, and in passing to the periphery they penetrate the meninges, which contribute to and are continuous with the connective-tissue sheaths investing them. Each nerve is attached by two roots, an afferent or dorsal root, which enters the cord along its postero-lateral sulcus, and an efferent or ventral root, which makes its exit along the ventro-lateral aspect.

With its inequalities in thickness and its conical termination the spinal cord is subdivided into four parts or regions: - (1) The cervical portion, with eight pairs of cervical nerves; (2) the thoracic portion, with twelve pairs of thoracic nerves; (3) the lumbar portion, with five pairs of lumbar nerves; and (4) the conus medullaris, or sacral portion, with five pairs of sacral and one pair of coccygeal nerves. From the termination of the conus medullaris, the pia mater continues below in the subarachnoid space into the portion of the vertebral canal not occupied by the spinal cord, and forms the non-nervous, slender, thread-like terminus, the filum terminale. This becomes continuous with the dura mater at its lower extremity.

In the early fetus the spinal nerves pass from their attachment to the spinal cord outward through the intervertebral foramina at right angles to the long axis of the cord, but, owing to the fact that the vertebral column increases consider- ably in length after the spinal cord has practically ceased growing, the nerve-roots become drawn caudal from their points of attachment, and, as is necessarily the case, their respective foramina are displaced progressively downward as the termination of the cord is approached, until finally the roots of the lumbar and sacral nerves extend downward as a brush of parallel bundles considerably below the levels at which they are attached. This brush of nerve-roots is the Cauda equina. The dura mater, being more closely related to the bony wall of the canal than to the spinal cord, extends with the vertebral column and thus envelops the Cauda equina, undergoing a slightly bulbous, conical dilation which decreases rapidly and terminates in the attenuated canal of the coccyx as the coccygeal ligament.

1.1. The enlargements

Wherever there is a greater mass of tissue to be innervated, the region of the nervous system supplying such must of necessity possess a greater number of neurons. Therefore, the regions of the spinal cord associated with the skin and musculature of the regions of the superior and inferior limbs are thicker than the regions from which the neck or trunk alone are innervated. Thus in the lower cervical region the spinal cord becomes broadened into the cervical enlargement, and likewise in the lumbar region occurs the lumbar enlargement. The spinal nerves attached to these regions are of greater size than in other regions.

The cervical enlargement [intumescentia cervicalis] begins with the third cervical vertebra, acquires its greatest breadth (12 to 14 mm.) opposite the lower part of the fifth cervical vertebra (origin of the sixth cervical nerves), and extends to opposite the second thoracic vertebra. Unlike the lumbar enlargement, its lateral is noticeably greater than its dorso-ventral diameter.

The lumbar enlargement [intumescentia lumbalis] begins gradually with the ninth or tenth thoracic vertebra, is most marked at the twelfth thoracic vertebra (origin of the fourth lumbar nerves), and rapidly diminishes into the conus medullaris.

Both the lumbar and thoracic regions are practically circular in transverse section. Neither diameter of the lumbar is ever so great as the lateral diameter of the cervical enlargement. The thoracic part attains its smallest diameter opposite the fifth and sixth thoracic vertebrae (attachment of the seventh and eighth thoracic nerves.)

The enlargements occur with the development of the upper and lower limbs. In the embyro they are not evident until the limbs are formed. In the orangutan and gorilla the cervical enlargement is greatly developed; the ostrich and emu have practically none at all.

1.2. Surface of the spinal cord

The cord is separated into nearly symmetrical right and left halves by the broad anterior median fissure into which the pia mater is duplicated, and opposite this, on the dorsal surface, by the posterior median sulcus. Along the lower two-thirds of the cord this sulcus is shallowed to little more than a line which marks the position of the posterior median septum; in the medulla oblongata it opens up and attains the character of a fissure. Each of the two lateral halves of the cord is marked off into a posterior, lateral, and anterior division by two other longitudinal sulci. Of these, the postero-lateral sulcus occurs as a slight groove 2 to 3.5 mm. lateral from the posterior median sulcus, and is the groove in which the root filaments of the dorsal roots enter the cord in regular linear series. The ventral division is separated from the lateral by the antero-lateral sulcus. This is rather an irregular, linear area than a sulcus. It is from 1 to 2 mm. broad, and represents the area along which the efferent fibers make their exit from the cord to be assembled into the respective ventral roots. This area varies in width according to the size of the nerve-roots, and, like the postero-lateral sulcus, its distance from the mid-line varies according to locality, being greatest on the enlargements of the cord. In the cervical region, and along a part of the thoracic, the posterior division is subdivided by a delicate longitudinal groove, the postero-intermediate sulcus, which becomes more evident toward the medulla oblongata and represents the line of demarcation between the fasciculus gracilis and the fasciculus cuneatus. Occasionally in the upper cervical region a similar line may be seen along the ventral aspect close to the anterior median fissure. This is the antero-intermediate sulcus, forming the lateral boundary of the ventral cerebro-spinal fasciculus.

Collectively, the entire space between the posterior median sulcus and the line of attachment of the dorsal roots is occupied by the posterior funiculus; the lateral space between the line of attachment of the dorsal and that of the ventral roots, by the lateral funiculus; and the space between the ventral roots and the anterior median fissure, by the anterior funiculus. Each of these funiculi is subdivided within into its component fasciculi.

The dorsal and ventral nerve-roots are not attached to the cord as such, but are first frayed out into numerous thread-like bundles of axons which are distributed along their lines of entrance and exit. These bundles are the root filaments [fila radicularia] of the respective roots. The fila of the larger spinal nerves are fanned out to the extent of forming almost continuous lines of attachment, while in the thoracic nerves there are appreciable intervals between those of adjacent roots. Throughout, the intervals are less between the fila of the ventral than between those of the dorsal roots.


2. Internal Structure of the Spinal Cord

By reflected light masses of medullated axons appear white in the fresh, and such masses are known as white substance. The spinaal cord consists of a continuous, centrally placed column of grey substance surrounded by a variously thickened tunic of white substance. The closely investing pia mater sends numerous ingrowths into the cord, bearing blood-vessels and contributing to its internal supporting tissue. The volume of white and of grey substance varies both absolutely and relatively at different levels of the cord. The absolute amount of grey substance increases with the enlargements. The absolute amount of white substance also increases with the enlargements coincident with the greater amount of grey substance in those regions. The relative amount of white substance increases in passing from the conus medullaris to the medulla oblongata, due to the fact that the ascending and descending axons associating the cord with the encephalon are the one contributed to the cord and the other gradually terminating in it at different levels along its entire descent.

2.1. The grey substance

In the embryo all the nerve-cells of the grey substance are derived from the cells lining the neural tube, and in the adult the column of grey substance, though greatly modified in shape, still retains its position about the central canal. In transverse section the column appears as a grey figure of two laterally developed halves, connected across the mid-line by a more attenuated portion, the whole roughly resembling the letter H. The cross-bar of the H is known as the grey commissure. Naturally, it contains the central canal, which is quite small and is either rounded or laterally or ventrally oval in section, according to the level of the cord in which it is examined. The canal continues upward, and in the medulla oblongata opens out into the fourth ventricle. Downward, in the extremity of the conus medullaris, it widens slightly and forms the rhomboidal sinus or terminal ventricle, then is suddenly constricted into an extremely small canal extending a short distance into the filum terminale, and there ends blindly. The grey commissure always lies somewhat nearer the ventral than the dorsal surface of the cord, and itself contains a few medullated axons which vary in amount in the different regions of the cord. The medullated axons crossing the mid-line on the ventral side of the central canal form the ventral or anterior white commissure ; those, usually much fewer in number, crossing on the dorsal side of the central canal, form the dorsal or posterior white commissure. These two commissures comprise fibers crossing in the grey substance as distinguished from others which cross in the white substance dorsal and ventral to them. The axons of these commissures serve in functionally associating the two lateral halves of the grey, column.

Each lateral half of the grey column presents a somewhat crescentic or comma- shaped appearance in transverse section, which also varies at the different levels of the cord. At all levels each half presents two vertical, well-defined horns, themselves spoken of as columns of grey substance. The dorsal horn [columna posterior] extends posteriorly and somewhat laterally toward the surface of the cord along the line of the postero-lateral sulcus. It is composed of an apex and a neck [cervix columnse posterioris].

In structure the apex is peculiar. The greater portion of it consists of a mass of small nerve-cells and neuroglia tissue, among which a gelatinous substance of questionable origin predominates, giving the horn a semi-translucent appearance. This is termed the gelatinous substance of Rolando, to distinguish it from a similar appearance immediate about the central canal, the central gelatinous substance. The apex of the dorsal horn is widest in the regions of the enlargements, especially the lumbar, and the gelatinous substance of Rolando is most marked in the cervical region. In these regions the cervix consists of a slight constriction of the dorsal horn between the apex and the line of the grey commissure. In the thoracic region, however, the base of the cervix is the thickest part of the dorsal horn. This thickness is due to the presence there of the nucleus dorsalis, or Clarke's column - a column of grey substance containing numerous nerve-cells of larger size than elsewhere in the dorsal horn, and extending between the seventh cervical and third lumbar segments of the cord. Tapering finelj' at its ends, this nucleus attains its height in the lower thoracic or first lumbar segment. About the ventro-lateral periphery of the nucleus dorsalis are scattered nerve-cells of the same type as contained in it. These cells are sometimes distinguished as Stilling's nucleus, though Clarke's column was also described by Stilling. They are more numerous about the lower extremity of the nucleus dorsalis, and they continue to appear below its termination in the lumbar region.

The ventral horn [columna anterior] of each lateral half of the grey figure is directed ventrally toward the surface of the spinal cord, pointing toward the antero-lateral sulcus. It contains the cell-bodies which give origin to the efferent or ventral root axons, and these axons make their emergence from the spinal cord along the antero-lateral sulcus. The ventral horns vary markedly in shape in the different regions. In certain segments each ventral horn is thickened later- ally and thus presents its two component columns of grey substance : the lateral horn [columna lateralis], a triangular projection of grey substance into the surrounding white substance, in line with or a little ventral to the line of the grey commissure; and the ventral horn proper [columna anterior], projecting ventrally. In the mid-thoracic region the lateral horn is relatively insignificant, and the anterior horn is quite slender; in the cervical and lumbar enlargements both horns are considerably enlarged.

The grey substance is not sharply demarcated from the white. In the blending of the two there are often small fasciculi of white substance embedded in the grey, and likewise the grey substance sends fine processes among the axons composing the white substance. Such processes or grey trabeculse are most marked along the lateral aspects of the grey figure and present there the appearance known as the reticular formation. The reticular formation of the spinal cord is most evident in the cervical region.

2.2. Minute structure

The large cell-bodies of the ventral horn as a whole are divisible into four groups, only three of which are to be distinguished in the mid-thoracic region of the spinal cord: - (1) A ventral group of cells, sometimes separated into a ventro-lateral and a ventro- medial portion, occupies the ventral horn proper, is constant throughout the entire length of the cord, and contributes axons to the ventral root, most of which probably supply the muscles adjacent to the vertebral column; (2) a dorso-medial group of cells, situated in the medial part of the ventral horn, just below the level of the central canal, gives origin to axons some of which go to the ventral root of the same side, but most of which cross the mid- line vi& the anterior white commissure, either to pass out in the ventral root of the opposite side or to enter the white substance of that side and course upward or downward, associating with other levels of the cord. Some of its axons terminate among the cells of the ventral horn in the same level of the opposite side; (3) a lateral group of cells, sometimes separated into a dorso- lateral and a ventro-lateral portion, occupies the lateral column or horn, and is best differentiated in the cervical and lumbar enlargements. Most of the axons arising from its larger cells are contributed to the ventral root of the same side, and such axons probably supply the muscles of the extremities. Some of those from its ventral portion are distributed to the muscles of the body-wall; the dorso-lateral portion is that part of the lateral column which persists throughout the cord, and is considered as supplying the visceral efferent fibers in the ventral roots. (4) an intermediate group, occupying the mid-dorsal portion of the ventral horn. Axons arising from its cells are probably seldom contributed to the ventral root, but instead course wholly within the central nervous system. Some pass to the opposite side of the cord, chiefly via the anterior and possibly the posterior white commissure, to terminate either in the same or different levels of the grey column. Others of longer course pass to the periphery of the cord, join one of the spino-cerebellar fasoicuh, and pass upward to the cerebellum.

Furthermore, there are scattered throughout the grey substance many smaller cell-bodies of neurons. These give rise to axons of shorter course, either commissural or associational proper. Of such axons many are quite short, coursing practically in the same level as that in which their cells of origin are located, and serve to associate the different parts of the grey sub- stance of that level. Others course varying distances upward and downward for the association of different levels of the grey column.

It is evident from the above that in addition to the various nerve-cells it contains, there is also to be found a felt-work of axons in the grey substance. Many of these axons are medullated, though not in sufficient abundance to destroy the grey character of the substance. The felt-work is composed of three general varieties of fibers: - (1) The terminal branches of axons entering from the fasciculi of the white substance and forming end-brushes about the various cell-bodies in the grey substance (partly medullated) ; (2) axons given off from the cells of the grey substance and which pass into the surrounding white substance either to enter the ventral-roots or to join the ascending and descending fasciculi within the spinal cord (partly medullated); (3) axons of Golgi neurons of type 11, which do not pass outside the confines of the grey substance (non-medullated). Some axons of any of these varieties may cross the mid-line and thus become commissural. In general all fibers of long course acquire medullary sheaths a short distance from then- cells of origin, and lose them again just before termination.

The white substance of the spinal cord. - The great mass of the axons of the spinal cord course longitudinally and form the thick mantle surrounding the column of grey substance. This mantle is divided into right and left homo- lateral halves by the anterior median fissure along its ventral aspect, and along its dorsal aspect by the posterior median septum, which is for the most part a connective-tissue partition derived from the pia mater along the line of the posterior median sulcus. The mantle is supported internally by interwoven neuroglia and white fibrous connective tissue, the latter, derived chiefly from the pia mater, closely investing it without.

The axons of the white substance belong to three general neuron systems:

Both the first and second systems increase in bulk as the cord is ascended. The ascending axons of each system are contributed to the white substance of the cord along its length, and therefore accumulate upward; the axons descending from the encephalon are distributed to the different levels of the cord along its length, and therefore diminish downward.

The mass of the third system of axons varies according to locality. Wherever there is a greater mass of neurons to be associated, as there is in the enlargements of the cord, a greater number of these axons is required. Their cells of origin, being in the grey substance of the cord, contribute to its bulk and thus both the cells and the axons of this system serve to make the enlargements more marked. In the lumbar and sacral regions the greater mass of the entire white substance consists of axons belonging to this system. It forms a dense felt-work about the grey column throughout the cord. Necessarily this system contains axons of various lengths. Some merely associate different levels within a single segment of the cord; others associate the different segments with each other. Axons which associate the structures of the spinal cord with those of the medulla oblongata may be included in this system. Many of these axons cross the mid- line both in the grey and in the white substance to associate the neurons of the two sides of the grey column. For purposes of distinction, such as cross the mid- line are called commissural fibers, while those which course upward and down-ward on the same side are association fibers. Coursing in longitudinal bundles about the grey figure, the latter compose the fasciculi proprii or ' ground bundles ' of the spinal cord.


3. Methods by which the conduction paths have been determined

A purely anatomical examination of a normal adult cord, prepared by whatever means, gives no indication of the fact that the mass of longitudinally coursing fibers of the white substance is composed of more or less definite bundles or fasciculi, each having a definite course, and whose axons form links (conduction paths) in a definite system of neuron chains.

Present information as to the size, position, and connections of the various fascicuH is based , upon evidence obtained by three different lines of investigation: -

3.1. (1) Physiological investigation

- (a) Direct stimulation of definite bundles or areas in section and carefully noting the resulting reactions which indicate the function and course of the axons stimulated, (b) 'Wallerian degeneration' and the application of such methods as that of Marchi. When an axon is severed, that portion of it which is separated from its parent cell-body degenerates. Likewise a bundle of axons severed from their cells of origin, whether by accident or design, will degenerate from the point of the lesion on to the locality of their termination in whichever direction thisimay be. This phenomenon was noted by Waller in 1852 and is known as WaUerian degeneration. By the application of a staining technique which is differential for degenerated or degenerating axons and a study of serial sections con- taining the axons in question, their course and distribution may be determined. The locality of their cells of origin, if unknown, may be determined by repeated experiment till a point of lesion is found not followed by degeneration of the axons under investigation, (c) The axonic reaction or 'reaction from a distance.' Cell-bodies whose axons have been severed undergo chemical change and stain differently from those whose axons are intact. Thus cell-bodies giving origin to a bundle of severed axons may be located in correctly stained sections of the region containing them.

3.2. (2) Embryological evidence

- In the first stages of their development axons of the cere- bro-spinal nervous system are non-medullated. They acquire their sheaths of myelin later. Axon pathways forming different chains become medullated at different periods. Based upon this fact a method of investigation originated by Flechsig is employed, by which the posi- tion and course of various pathways may be determined. A staining method differential for medullated axons alone is apphed to the nervous systems of foetuses of different ages, and path- ways meduUated at given stages may be followed from the locality of their origin to their termination. In the later stages, when most of the pathways are medullated and therefore stain alike, the less precocious pathways may be followed by their absence of medullation.

3.3. (3) Direct anatomical evidence

- (a) Stains differential for axons alone are applied to a given locality to determine the fact that the axons of a given bundle actually arise from the cell-bodies there, or that axons traced to a given locality actually terminate about the cell-bodies of that locality. For example, it may be proved anatomically that the axons of a dorsal root arise from the cells of the corresponding spinal ganglion, and then these axons may be traced into the spinal cord and their terminations noted either by collateral or terminal twigs, or the fasciculus they join in their cephalic course may be determined. (b) The staining properties and the size and distribution of the tigroid masses in the cell-bodies of sensory neurons differ from those in the motor neurons, and recently Malone has claimed that, in the central system, the cell-bodies in the nuclei of sensory neuron chains, those ascending toward the cerebral cortex, may be distinguished from the cell-bodies of the motor or descending chains by the arrangement and size of their tigroid masses. He claims further that in the same way, the cell- bodies of the somatic efferent neurons may be distinguished from those of the visceral efferent neurons. In this way the locality of origin of certain physiologically known paths may be determined.

3.4. (4) The so-called pathologico-anatomical method

It is based upon the same general principles as is the physiological (or experimental) method. A pathological lesion, a local infection or a tumor for example, may destroy a nucleus of cell-bodies or sever a bundle of axons, and the resulting degeneration of the axons may be followed through serial sections suitably prepared. The locality of the lesion known, the path may be followed to determine the locality of its termination; its locality of termination known from the symptoms resulting, the path may be followed to its cells of origin, or to determine whatever be the locality of the lesion.


4. Funiculi

In order that the various fasciculi may be referred to with greater ease, the white substance of the spinal cord in section is divided into three areas known as funiculi or columns and which correspond to the funiculi already mentioned as evident upon the surface of the cord when intact. The funiculi are outlined wholly upon the basis of their position in the cord and with reference to the median line and the contour of the column of grey substance; their component fascicul are defined upon the basis of function. (1) The posterior funiculus or column is bounded by the posterior median septum and the line of the dorsal horn; (2) the lateral funiculus or column is bounded by the lateral concavity of the grey column and the lines of entrance and exit of the dorsal and ventral roots; (3) the ventral funiculus or column is bounded by the Hne of exit of the ventral roots, and by the anterior median fissure.

4.1. The posterior funiculus or column [funiculus posterior].

This funiculus is composed of two general varieties of axons arranged in five fasciculi. First, and constituting the predominant type in all the higher segments of the cord, are the afi'erent or general sensory axons, which arise in the spinal ganglia, enter the cord by the dorsal roots, assume their distribution to the neurons of the cord, and then take their ascending course toward the encephalon. The axon of the spinal ganglion neuron undergoes a T-shaped division a short distance from the cell- body, one limb of this division terminating in the peripheral organs and the other going to form the dorsal root. Upon entering the cord the dorsal root axons undergo a Y-shaped bifurcation in the neighborhood of the dorsal horn, one branch ascending and the other descending. Their ascending branches form the fasciculus gracilis (Goll's column) and the fasciculus cuneatus (Burdach's column). These fasciculi are the chief ascending or sensory spino-cerebral connections, the direct sensory path to the brain. The neurons represented in them constitute the first link in the neuron chain between the periphery of the body- and the cerebral cortex.

In threading their way toward the brain, these sensory axons tend to work toward the midline. Therefore those of longer course are to be found nearer the posterior septum, in the upper segments of the cord, than those axons which enter the cord by the dorsal roots of the upper segments. Thus it is that the fasciculus gracilis, the medial of the two fasciculi, contains the axons which arise in the spinal ganglia of the sacral and lumbar segments. In other words, it is the fasciculus bearing sensory impulses from the lower limbs to the brain, while the fasciculus cuneatus, the lateral of the two, is the corresponding pathway for the higher levels. Naturally, there is no fasciculus cuneatus as such in the lower segments of the spinal cord. The axons being much blended at first, it is only in the upper thoracic and cervical region that there is any anatomical demarcation between the two fasciculi. In this region the two become so distinct that there is in some cases an apparent connective-tissue septum between them, continuing inward from the postero-intermediate sulcus - the surface indication of the line of their junction.

Upon reaching the medulla oblongata the fibers of the fasciculus gracilis and the fasciculus cuneatus terminate about cells grouped to form the nuclei of these fasciculi. The nucleus of the fasciculus gracilis is situated medially and begins just below the point at which the central canal opens into the fourth ventricle; the nucleus of the fasciculus cuneatus is placed laterally and ex- tends somewhat higher than the other nucleus. The neurons whose cell-bodies compose these nuclei constitute the second links in the neuron chains conveying sensory impulses from the periphery to the cerebral cortex.

The descending or caudal branches of the dorsal root axons are concerned wholly with the neurons of the spinal cord. They descend varying distances, some of them as much as four segments of the cord, and give off numerous collaterals on their way to the cells of the grey column. Those terminating about cell-bodies of the ventral horn which give rise to the ventral or motor root-fibers, are responsible for certain of the so-called 'reflex activities' and thus contribute to the simplest of the reflex arcs. In descending they serve to associate different levels of the grey substance of the cord with impulses entering by way of a single dorsal root. Some of their collaterals cross the mid-line in the posterior white commissure, and thus become connected with neurons of the opposite side. The caudal branches of longer course are scattered throughout the ventral portion of the fasciculus cuneatus (middle root zone), and the longest show a tendency to collect along the border-line between the fasciculus cuneatus and the fasciculus gracilis, and thus contribute largely to the comma-shaped fasciculus. Also some of the longest of them in the lower levels course in the oval bundle or septo-marginal root zone.

The ascending branches of the dorsal root axons also give off collaterals to the grey sub- stance of the cord, thus extending the area of distribution of a given dorsal nerve-root to levels of the cord above the region at which the root enters.

The greater number of the terminations of dorsal root axons within the spinal cord are concerned first with neurons other than those contributing ventral root-fibers. The greater mass of the neurons concerned are those of the Golgi type II and those contributing the fasciculi proprii or ground bundles of the spinal cord, or the second variety of axons composing the posterior funiculus. The latter fasciculi arise from the smaller cells of the grey column.

These axons pass from the grey substance to enter the surrounding white substance, bifurcate into ascending and descending branches, which in their turn give off numerous collaterals to the cells of the grey substance of the levels through which they pass. The cell-bodies giving origin to such axons are so numerous that the entire column of grey substance is surrounded by a continuous felt-work of axons of this variety.

The dorsal fasciculus proprius (anterior root zone of posterior column) arises chiefly from cells situated in the dorsal horn (stratum zonale). Coincident with the ingrowth and arrangement of the fasciculi gracilis and cuneatus many fibers of the dorsal fasciculus proprius go to form both the oval bundle and the comma-shaped fasciculus. Thus these two bundles are mixed, being fasciculi proprii which contain caudal branches of dorsal root axons. The association fibers in the oval bundle are the longest of any belonging to the dorsal fasciculus proprius. The cephalic and caudal branches combined of some are said to extend more than half the length of the cord and it has been claimed that some even associate the cervical region with the conus medullaris. Based upon this claim, Obersteiner has called the oval bundle, the "dorso-medial sacral field" and Edinger has referred to the most dorsal part of it as the "tractus cervico-lumbalis dorsalis." The 'median triangle' is formed by the continuation of the dorsal fasciculi proprii with the oval or septo-marginal fasciculus. Some of the axons of the dorsal fasciculus proprius cross the midline to distribute impulses to the neurons of the opposite side. These commissural axons, together with certain collaterals of the dorsal root axons, which cross the mid-line out- side the dorsal white commissure, compose the so-called cornu-commissural tract at the base of the posterior septum.

4.2. The lateral funiculus or column [funiculus lateralis]

Not all the axons of the posterior or dorsal nerve-roots extend to the encephalon. Estimation shows that the sum of all the dorsal roots is greatly in excess of the sum contained in the fasciculi cuneatus and gracilis just before these enter their nuclei of termination. Therefore many of the ascending dorsal root axons are concerned with spinal- cord relations wholly.

The marginal zone of Lissauer, situated along the lateral margin of the postero-lateral sulcus is composed largely of dorsal root axons. Many of these finally work across the line of the sulcus into the posterior funiculus. Many of the dorsal root-fibers which do not reach the brain occur in Lissauer's zone. Many others of course occur throughout the posterior column. Lissauer's zone also contains some fibers arising from the small cells of the dorsal horn, and to this extent corresponds to a fasciculus proprius. Ranson has found that large numbers of the non-medullated dorsal root axons which enter the cord are contributed to Lissauer's zone.

The lateral fasciculus proprius (lateral ground bundle, lateral limiting layer) is situated in the lateral concavity of the grey column and is continuous with the other fasciculi proprii both dorsal and ventral. Beyond that it probably contains fewer commissural axons, it is of the same general significance as the others. It is frequently divided into small bundles by the reticular formation. The lateral cerebro-spinal fasciculus (crossed pyramidal tract). In contrast to the sensory fibers passing through the spinal cord conveying impulses destined to reach the cerebral cortex, axons are given off from the pyramidal cells of the cortex, which descend to terminate about the cells of the grey substance of the spinal cord, chiefly the cells which give origin to the ventral root-fibers.

Upon reaching the medulla oblongata in their descent, these axons are accumulated into two well-defined, ventrally placed bundles, the pyramids, one from each cerebral hemisphere. In passing through the brain stem the pyramids contribute many fibers which cross the mid-line to terminate in the motor nuclei of the cranial nerves of the opposite side, and thus decrease appreciably in bulk. According to the estimate of Thompson, only about 160,000 of the pyramidal fibers are destined to enter the spinal cord.

Upon reaching the lower part of the medulla, the greater mass of the fibers of each pyramid, which are destined to enter the cord, suddenly cross the mid-line in the 'decussation of the pyramids.' The remainder retain their ventral position in their descent decussating gradually in the cord itself. The pyramidal fibers which cross in the medulla course in the lateral column ventral to Lissauer's zone, and lateral to the lateral fasciculus proprius, and form the lateral cerebrospinal fasciculus (crossed pyramidal tract). It is a large fasciculus, oval shaped in transection, and since its axons terminate in the grey column of the cord all along its length, it decreases in bulk as the cord is descended.

In addition to the three dispositions of the dorsal root axons given above, certain of them, either by collaterals or terminal twigs, form telodendria about the cells of the dorsal nucleus (Clarke's column), which nucleus extends from about the seventh cervical to the third lumbar segment of the cord. The axons given off by these cells pass to the dorso-lateral periphery of the lateral funiculus, and there collect to form the dorsal spino-cerebellar fasciculus (direct cerebellar tract of Flechsig). As such they ascend without interruption, and in the upper level of the medulla oblongata pass into the cerebellum by way of the inferior cerebellar peduncle or restiform body. Necessarily, this fasciculus is not evident in levels below the extent of the nucleus dorsalis.

Also situated superficially in the lateral funiculus is another ascending con- duction path, and, like the dorsal spino-cerebellar fasciculus, to which it is adjacent, it is also in part at least a cerebellar connection. Its position suggests its name, superficial ventro-lateral spino-cerebellar fasciculus (Gowers' tract).

This tract at 'present' does not include as great an area in transverse section as when originally described. The more internal portion of the original Gowers' tract is now given a separate significance, and will be considered separately. While the exact location in the grey column of all the cell-bodies giving origin to the superficial ventro-lateral spino-cerebellar fasciculus is un-certain, it is known that certain ventral horn cells contribute their axons to it. Many of its cells of origin are scattered in the area immediately ventral to the nucleus dorsalis, others in the intermediate and mesial portion of the lateral group of ventral horn cells. In the lumbar region these cells are quite numerous, and, therefore, the fasciculus begins at a lower level in the spinal cord than does the direct cerebellar tract. In degenerations it becomes visible in the upper segments of the lumbar region, and has been proved to increase notably in volume as the cord is ascended. Its axons arise for the most part directly from cell-bodies of the same side of the cord, though it has been shown by several investigators that many of its axons come from the grey substance of the opposite side by way of the ventral white commissure. Terminal twigs and collaterals of the dorsal root-fibers, mostly of the same side, but occasionally from the opposite side, terminate about its cells of origin. At one time Gowers' tract was considered an entity, but now, even in the more Umited area it occupies, it must be considered a mixture of axons of several terminal destinations or distinct neuron systems. The destination of some of its axons has not been determined with certainty. A portion, the spino-cerebellar fasciculus proper, go to the cerebellum, and there have been traced to the cortex of the superior vermis. Most of these reach the cerebellum not by way of the restiform body, as does the dorsal spino-cerebellar tract, but pass on in the brain-stem to the level of the inferior corpora quadrigemina, and there turn back to join the brachium conjunctivum or superior cerebellar peduncle. (Auerbach, Mott, Hoche.) Only a few of its axons leave the fasciculus lower down in the medulla, to enter the cerebellum by way of the restiform body, in company with the dorsal spino-cerebellar tract. (Rossolimo, Tschermak.) Another portion of its axons are thought to reach the cerebrum, probably the nucleus lentiformis, though it has not been positively traced further than the superior corpora quadrigemina. Many axons in Gowers' tract of the cord correspond to those of the fasciculi proprii, and merely run varying distances in the cord, to turn again into its grey substance. Schaeffer followed some of these from the lumbar region up to the level of the second cervical nerve.

In the ventro-medial border of Gowers' tract and immediately upon the periphery, near the antero-lateral sulcus (exit of ventral nerve-roots), there is found in the higher segments of the cord a small oval bundle, the spino-olivary fasciculus or Helweg's (Bechterew's) bundle. The functional direction of its fibers has not been settled.

It is asserted to arise from cell-bodies of the ohve in the medulla oblongata, and in the cord is believed to be associated with the cells of the ventral column of grey substance, probably those of the lateral; horn. More recent claims assert that it arises from cell bodies in the cord and thus is spino-olivary. By some observers it has been traced as far down as the mid-thoracic region; by others, however, only as far as the third cervical segment. The olives being nuclei largely concerned with cerebellar connections, Helweg's fasciculus is probably an indirect cerebellar association with the spinal cord neurons. It is composed of axons of relatively very small diameter, and is one of the last fasciculi of the spinal cord to become medullated.

Situated between the superficial ventro-lateral spino-cerebellar fasciculus and the lateral fasciculus proprius is an area which, in transverse sections, may be, by position, referred to collectively as the intermediate fasciculus. So intermingled are the axons comprising it that it has been called the mixed lateral zone. It contains fibers of at least five functional varieties:

(1) Fibers belonging to the lateral fasciculus proprius which are of longer extent gradually course farther away from the grey substance of the cord and such mix into the intermediate fasciculus.

(2) It is said to contain fibers descending from the cerebellum to associate with the neurons of spinal cord, probably directly with the ventral root or motor neurons.

(3) The rubro-spinal fasciculus. - This arises from cell-bodies in the red nucleus of the tegmentum (in the mesencephalon) and is a crossed fasciculus. Axons arising from the red nucleus of one side cross the mid-Une while yet in the mesencephalon and descend in the lateral funiculus of the cord to terminate gradually about cell-bodies of the ventral horn, both those which give rise of ventral root fibers and those which contribute to the fasciculi proprii. Its fibers are more thickly bundled in a crescentic area fitting onto the ventral side of the lateral cerebro-spinal fasciculus, and some are said to mix into the area of this latter.

(4) The vestibulo-spinal fasciculus. - This is sometimes called the lateral vestibulo-spinal fasciculus from the fact that there is a tract of similar significance in the ventral funiculus of the cord. It arises from some of the cell-bodies comprising Deiter's nucleus, the lateral nucleus of termination of the vestibular nerve, and from some of those of the spinal nucleus (nucleus of the descending root) of this nerve, all of which is in the medulla. It descends the cord, un- crossed, to terminate gradually about ventral horn cells, thus comprising a part of the apparatus for the equilibration of the body. . Its fibers are thought to be more closely collected in the area immediately ventral to the rubro-spinal fasciculus, but of course commingle with the latter.

(5) The corpora-quadrigemina-thalamus path. The most lateral portion of the intermediate fasciculus, a small area once included in Gower's tract, contains fibers both ascending and descending, connecting the spinal cord with the thalamus (diencephalon) and the quadrigeminate bodies of the mesencephalon. These are crossed paths. The ascending fibers arise from cell-bodies in the ventral horn of one side, cross in the ventral white commissure (commissural neurons) and course upward in the intermediate fasciculus to their termination in the opposite side. Those terminating about cell-bodies in the thalamus form what is known as the spino- thalamic tract, while those terminating in the nuclei of the quadrigeminate bodies are called the spino-mesencephalic or spino-tectal tract (Iradus spino-tectalis) . It is not known in which region of the cord most of these fibers arise but it is quite probably the cervical region. The fibers which arise from cell-bodies of the thalamus and nuclei of the quadrigeminate bodies cross the mid-line in the mesencephalon and descend the cord to terminate gradually about cell-bodies in the ventral horn of the opposite side. Those from the thalamus are known as the thalamo-spinal tract and those from the quadrigemina, as the mesencephalo- or tecto-spinal tract. The latter is thought to be the larger.

By the fibers of the above tracts general sensory impulses from the body (skin, etc.) are carried to the central portion of the optic apparatus, and the descending fibers give a simple anatomical possibility for the movements of the body in response to visual and auditory im- pulses. The descending fibers are thought to terminate chiefly in contact with association neurons of the fasciculi proprii, these transferring the impulses to the neurons giving origin to the ventral or motor root fibers, but some are thought to terminate directly about the cell- bodies of ventral-root neurons. A portion of the intermediate fasciculus, most adjacent to Gower's tract, has been designated as Loewenthal's tract.

4.3. The anterior funiculus or column [funiculus anterior]

The intermediate fasciculus is continued ventrally and medially across the line of exit of the ventral root axons, and thus into the anterior funiculus. This portion is also mixed, but its axons of long course associate somewhat different portions of the nerve axis from those connected by the more lateral portion.

According to the studies of Flechsig, von Bechterew, and Held, this mesial portion contains fibers, both ascending and descending, which associate the various levels of the grey substance of the spinal cord with the neurons in the reticular formation of the medulla oblongata. The levels to which they have been traced comprise the olivary nuclei, which are largely concerned in cerebellar connections, and the nuclei of the vagus, glosso-pharyngeal, auditory, facial and the spinal tract of the trigeminus. Also some of the ascending fibers are probably associated with the nuclei of the eye-moving nerves. This portion of the intermediate fasciculus also grades into and is mixed with the axons of the ventral fasciculus proprius, as is its lateral portion with the lateral fasciculus proprius. In other words, the fasciculi proprii proper, the axons nearest the grey substance, serve for the intersegmental association of the different levels of the grey substance of the cord, while the intermediate fasciculus contains axons of longer course which serve to associate more distant levels of the grey substance of the nerve axis - that of the spinal cord with its upward continuation into the medulla oblongata, pons and mesencephalon.

4.4. The anterior marginal fasciculus, ventral vestibulo-spinal tract (Loewenthal's tract)

It forms the superficial boundary of the mesial portion of the intermediate fasciculus. It is a narrow band, parallel with the surface of the cord, and extends medially from the medial extremity of Gowers' tract (from Helweg's bundle) to the beginning of the anterior median fissure.

The axons belonging to it proper are descending from the recipient nuclei of the vestibular nerve. Of these nuclei it has been held by some investigators that only Deiters' nucleus (the lateral nucleus of termination in the upper extremity of the medulla oblongata) gives origin to the axons of the anterior marginal fasciculus. Others agree with Tschermak that the superior and more laterally situated Bechterew's nucleus of the vestibular nerve also contributes axons to it, and quite probably the nucleus of the spinal root of the vestibular adds further axons. Still other investigations have shown that a part at least of the fasciculus comes from the nucleus fastigius (roof nucleus) of the cerebellum. Since many axons from both Deiters' and Beehterew's nucleus terminate in the nucleus fastigius, the ventral vestibulo-spinal fasciculus is, in any case, a conduction path from the nerve connections for equilibration to the grey sub- stance of the spinal cord. The fasciculus is said to extend as far as the sacral region of the cord, its axons terminating about the cells of the ventral horns. The term "ventral" is added to its name to distinguish it from the vestibulo-spinal tract described above as coursing in the lateral funiculus. It is considered an uncrossed pathway.

The ventral cerebro-spinal fasciculus (anterior or direct pyramidal tract), as stated above, is the uncrossed portion of the descending cerebro-spinal system of neurons. It is a small, oblong bundle, situated medially in the anterior funiculus, parallel with the anterior median fissure. Like the lateral cerebro-spinal fasciculus (crossed pyramidal tract), its axons arise from the large pyramidal cells of the motor area of the cerebral cortex, and transmit their impulses to the neurons of the ventral horns of the grey substance of the spinal cord, and almost entirely to those neurons which give origin to the ventral or motor root fibers.

It represents merely a delayed decussation of the pyramidal fibers, for instead of crossing to the opposite side in the lower portion of the medulla oblongata, as do the fibers of the lateral fasciculus, its fibers decussate all along its course, crossing in the ventral white commissure and in the commissural bundle of the cord to terminate about the ventral horn cells of the opposite side. Hoohe, employing Marchi's method, found that a few of its fibers terminate in the ventral horn of the same side. This conforms to the pathological and experimental evidence that there are homolateral or uncrossed fibers in the crossed pyramidal tracts also. Like the crossed tract, the ventral pyramidal tract diminishes rapidly in volume as it descends the cord. Its loss is greatest in the cervical enlargement, and it is entirely exhausted in the thoracic cord. With the exception of the anthropoid apes and certain monkeys, none of the mammalia below man, which have been investigated, possess this ventral pyramidal tract

Lying between the ventral cerebro-spinal fasciculus and the pia mater of the anterior median fissure is a thin tract of descending axons continuous ventrally with the anterior marginal fasciculus. From its position it is known as the sulco-marginal fasciculus; functionally it is the ventral mesencephalo-spinal (tecto- spinal) tract.

The extent of its course in the spinal cord is uncertain. It arises from the cells of the grey substance of the superior pair of the quadrigeminate bodies, and there, in largest part at least, it crosses the mid-line, and in the so-called 'optic acoustic reflex path' descends through the medulla oblongata into the spinal cord of the opposite side. The superior quadrigeminate bodies having to do with sight, this tract forms a second path conveying visual impulses to the neurons of the spinal cord.

4.5. The commissural bundle

It is situated about the floor of the anterior median fissure, and is the most dorsal tract of the anterior funiculus. It contains decussating or commissural axons of three varieties.

(1) It contains the decussating axons of the ventral cerebro-spinal fasciculus throughout the extent of that fasciculus;

(2) it is chiefly composed of the axons of the ventral fasciculus proprius which arise in the grey substance (ventral horn) of one side, cross the mid-line as com- missural fibers, and course both upward and downward to be distributed to the neurons of different levels of the grey substance of the opposite side;

(3) it contains decussating axons which arise from cell-bodies in the grey substance of one side and cross the mid-line to terminate about cell-bodies in practically the same level of the opposite side. The latter are merely axons belonging to the ventral white commissure which course without the confines of the grey figure. The commissural bundle is present throughout the length of the spinal cord, and is largest in the enlargements, i. e., where the association and commissural neurons occur in greater number generally. In its two last-mentioned varieties of axons it corresponds to the commissural portion of the dorsal fasciculus proprius (the cornu-commissural bundle).

The ventral fasciculus proprius is but a continuation of the lateral fasciculus proprius, and is composed of ascending and descending association fibers of the same general significance.


5. Summary of the spinal cord

The spinal cord contains two general classes of axons arranged into three general systems. It contains axons which - (a) enter it from cell-bodies situated outside its boundaries, i. e., in the spinal ganglia and in the encephalon, and (b) axons which arise from cell-bodies situated within its own grey substance, some of which axons pass outside its boundaries both to the periphery and into the encephalon; some of which remain wholly within it. Its axons comprise

For these relations the grey substance of the cord contains three general classes of nerve-cells: - those which give rise to the peripheral efferent or motor axons of the ventral roots; those which give rise to central axons of long course, going to the encephalon; and those which supply its central axons of shorter course, the association and commissural systems.

5.1. The three systems :

5.1.1. (1) Association and commissural

Axons of spinal ganglion (afferent) neurons bifurcate within the cord into cephalic and caudal branches which extend varying distances upward and downward and terminate, (a) about cell-bodies whose axons are short and terminate within the grey substance of the same side and in the same level as their cell- bodies (Golgi neurons of type II); (b) about cell-bodies whose axons pass without the grey sub- stance, bifurcate into cephalic and caudal branches to terminate in the grey substance of the same side but in various levels above and below (association fibers in the dorsal, lateral and ventral fasciculi proprii); (c) about cell-bodies whose axons cross the mid-line to terminate either in the same level of the grey substance of the opposite side, or bifurcate and the cephalic and caudal branches pass in the fasciculi proprii to terminate in various levels of the grey substance of the opposite side. The longer cephalic branches of (b) and (c) may terminate in the meduUa oblongata. All, associated with ventral root (efferent) neurons, belong to the neuron chains for the so-oaOed reflex activities.

5.1.2. (2) The cerebral system

(a) The cephalic branches of certain spinal ganglion neurons ascend beyond the bounds of the spinal cord to terminate within the medulla. Those ascending from the spinal ganglia of lower thoracic and lumbo-sacral segments accumulate mesiaUy to form the fasciculus gracilis which terminates in the nucleus of this fasciculus; those arising from the upper thoracic and cervical segments accumulate more laterally in the posterior funiculus to form the fasciculus cuneatus which terminates in the nucleus of the fasciculus cuneatus.

(b) The impulses transferred to the neurons of these nuclei are borne across the mid-line and finally reach the sensory-motor area of the cerebral cortex, and cell-bodies here give rise to axons which descend, some decussating in the medulla to form the lateral cerebrospinal fasciculus, others form the uncrossed ventral cerebrospinal fasciculus which crosses the mid-line as it descends the cord. Both of these fasciculi transfer their impulses either directly to efferent ventral horn neurons, or to association neurons and these to the efferent neurons,

(c) The cephalic and caudal branches of spinal ganglion neurons terminate about cell-bodies in the grey substance of the cord whose axons cross the mid-line and ascend laterally to terminate either in the quadrigeminate bodies (spino-mesencephalic tract), or in the thalamus (spino-thalamic tract),

(d) Cell-bodies in thalamus and superior quadrigeminate bodies (receiving optic im- pulses) and in the inferior quadrigeminate bodies (probably mediating auditory impulses), give axons which cross the mid-line in the mesencephalon and descend, forming the thalamo- spinal and mesencephalospinal tracts, to terminate in contact with the efferent neurons of the cord. Axons from both sources descend in the lateral funiculus, while from the superior quadrigeminate body, a separate bundle descends in the ventral funiculus as the sulco-marginal {ventral mesencephalospinal) fasciculus,

(e) The rubrospinal tract arises from cell-bodies in the red nucleus (in the mesencephalon), crosses the mid-line and descends in the lateral funiculus to transfer (probably cerebellar) impulses to the efferent neurons of the spinal cord.

5.1.3. (3) The cerebellar system

(a) The cephalic and caudal branches of spinal ganglion neurons give telodendria about the cell-bodies forming the dorsal nucleus of the cord (Clarke's column) and about cell-bodies situated in grey substances ventral to the dorsal nucleus ("Stilling's nucleus") and in the lateral horn. Axons arising from the cells of the dorsal nucleus pass laterally to form the dorsal spino-cerebellar fasciculus which ascends into the cerebellum by way of its inferior peduncle of the same side and terminates about cell-bodies of its cortex. Axons arising from Stilling's nucleus and the lateral horn cells, of both the same and opposite sides of the cord, accumulate to form the superficial ventro-lateral spino-cerebellar fasciculus, which ascends to enter the cerebellum by way of its superior peduncle and terminate about the cells of the cerebellar cortex,

(b) A few axons arising in the roof nucleus of the cerebellum probably descent in the animor marginal fasciculus in company with the ventral vestibulospinal tract to terminate upon the efferent neurons of the cord,

(c) The inferior olivary nucleus, in the medulla, is a cerebellar relay and its cell-bodies are associated with the neurons of the upper portion of the same side of the spinal cord. Whether the axons arise in the olivary nucleus or in the grey substance of the cord is uncertain, but the more usual supposition favors the cord and thus the name, spino-olivary fasciculus is given them,

(d) Among its other functions, the cerebellum is concerned with equilibration. The vestibular nerve is the afferent nerve of equilibration and a large mass of the axons arising from its nuclei of termination terminate in the cerebellum, in the roof nuclei especially. Axons arising from cell-bodies in Deiters' nucleus (its lateral nucleus of termination) and in the nucleus of its descending root descend the cord in the lateral funiculus to form the (lateral) vestibulospinal tract, and also in the anterior marginal fasciculus to form ventral vestibulospinal tract. Impulses borne by these axons reach the efferent or motor root neurons. The rubro-spinal fasciculus, mentioned above also may be possibly considered as belonging to the cerebellar system.


6. Sympathetic relations

The cell-bodies of the efferent neurons in the ventral horns are of two general varieties: (a) those whose axons terminate upon skeletal muscle (somatic efferent), and (b) those whose axons terminate in contact with cell-bodies of sympathetic neurons, the splanchnic or visceral efferent neurons. The axons of the sympathetic neurons, in their turn, terminate upon cardiac and smooth muscle (motor) and in glands (secretory). Like the somatic, the visceral efferent neurons receive impulses within the ventral horns (a) from the cephalic and caudal branches of spinal ganglion neurons, (b) the descending cerebro-spinal fasciculi, and (c) from either, by way of the fasciculi proprii and Golgi neurons of type II. Their cell-bodies are situated for the most part in the dorsal portion of the lateral horn (dorso-lateral group of cells), which is the only portion of the lateral horn present in the thoracic region of the cord. Many of the visceral efferent fibers leave the spinal nerves distal to the spinal ganglia and make the white communicating rami, thus going to the nearest sympathetic ganglia; others pass on in the spinal nerve and its branches to terminate in more distal sympathetic ganglia. Dogiel has described axons which arise in sympathetic ganglia and terminate upon the cell-bodies of the spinal ganglia. Such convey sensory impulses which, however, enter the spinal cord by way of the dorsal root branch of the spinal ganglion neuron. Such afferent sympathetic neurons are relatively rare, the peripheral distribution of the ordinary spinal ganglion neuron in the domain of the sympathetic supplying the needs for sensory axons.

In transverse sections of the spinal cord, the relative area of white substance as compared with that of grey increases as the cord is ascended. The absolute area of each varies with the locality, both being greatest in the enlargements. The grey substance predominates in the conus medullaris and lower lumbar segments. The white substance begins to predominate in the upper lumbar segments, not because of the increased presence of ascending and descending cerebral and cerebellar axons, but because of the increased volume of the fasciculi proprii coincident with the greater mass of grey substance to be intersegmentally associated in this region. In the thoracic region the greatly predominating white substance is composed mostly of the axons of long course. The greatly increased absolute amount of white substance in the cervical region is due both to the greater accumulation of cerebral and cerebellar axons in this region and to the increased volume of the fasciculi proprii of the cervical enlargement.

The axons of the spinal cord begin to acquire their myelin sheaths during the fifth month of intra-uterine life and myelinization is not fully completed till between the fifteenth and twentieth years. In general, axons which have the same origin and the same locality of termination - the same function - acquire their sheaths at the same time. While it has been proved that the medullary sheath does not necessarily precede the functioning of an axon, it may be said that those fasciculi which first attain complete and definite functional ability are the first to become medullated. At birth all the fasciculi of the spinal cord are medullated except Helweg's fasci- culus, and occasionally the lateral and ventral cerebro-spinal tracts. The latter tracts vary considerably and in general may be said to become medullated between the ninth month (before birth) and the second year. As indicated by their medullation, those axons by which the cord is enabled to function as an organ per se, that is, the axons making possible the simpler reflex activities, complete their development before those axons which involve the brain with the activities of the cord.

According to Flechsig and van Gehuohten, and investigators succeeding them, the following is the order in which the axons of the cord become medullated:

(1) The afferent and efferent nerve-roots and commissural fibers of the grey substance.

(2) The fasciculi proprii, first the ventral, then the lateral, and last the dorsal, fasciculus proprius.

(3) The fasciculus cuneatus (Burdach's column) and Lissauer's zone - the area of tho.se ascending spino-cerebral fibers which run the shorter course and which convey impulses from the upper limbs, thorax and neck.

(4) Fasciculus gracilis (GoU's column).

(5) The dorsal spino-cerebellar fasciculus (direct cerebellar tract).

(6) The superficial antero-lateral spino-cerebellar fasciculus (Gowers' tract).

(7) The lateral cerebro-spinal fasciculus (crossed pyramidal) and the ventral cerebro- spinal fasciculus (direct pyramidal tract).

(8) The spino-olivary or Helweg's (Beohterew's) fasciculus.

The axons descending from the cerebellum and the brain-stem are so mixed with other axons that it is difficult to determine the sequence of their medullation. The fasciculi contaia- ing them also contain axons of the variety in the fasciculi proprii and so show medullation early. It is probable that the ascending, spino-cerebellar, fibers acquire their myeUn earlier than the descending, if descending exist.


7. Blood Supply of the Spinal Cord

The spinal rami of the sacral, lumbar, intercostal, or vertebral arteries, as the case may be, accompany the spinal nerves through the intervertebral foramina, traverse the dura mater and arachnoid, and each divides into a dorsal and a ventral radicular artery. These accompany the nerve-roots to the surface of the cord, and there break up into an anastomosing plexus in the pia mater. From this plexus are derived three tortuously coursing longitudinal arteries and numerous independent central branches, which latter penetrate the cord direct. Of the longitudinal arteries, the anterior spinal artery zigzags along the anterior median fissure and gives off the anterior central branches, which pass into the fissure and penetrate the cord. These branches give of a few twigs to the white substance in passing, but their most partial distribution is to the ventral portion of the grey substance. The two posterior spinal arteries, one on each side, course near the lines of entrance of the dorsal root-fibers. They each branch and anastomose, so that often two or more posterior arteries may appear in section upon either side of the dorsal root. These give off transverse or central twigs to the white substance, but especially to the grey substance of the dorsal horns. Of the remaining central branches many enter the cord along the efferent fibers of the ventral roots, and are distributed chiefly to the grey substance; others from the peripheral plexus throughout penetrate the cord and break up into capillaries within the white substance. Some of the terminal twigs of these also enter the grey substance. The blood supply of the grey substance is so much more abundant than that of the white substance that in. injected preparations the outline of the grey figure may be easily distinguished by its abundance of capillaries alone. The central branches are of the terminal variety. In the white substance the capillaries run for the most part longitudinally, or parallel with the axons.

The venous system is quite similar to the arterial. The blood of the central arteries is collected into corresponding central venous branches which converge into a superficial venous plexus in which are six main longitudinal channels, one along the posterior median sulcus, one along the anterior median fissure, and one along each of the four lines of the nerve-roots. These comprise the posterior and anterior external spinal veins.

The internal spinal veins course along the ventral surface of the grey commissure, and arise from the convergence of certain of the twigs of the anterior central vein. The posterior central vein courses along the posterior median septum in company with the posterior central artery, and empties into the median dorsal vein. The venous system communicates with the coarser extra-dural or internal vertebral plexus chiefly by way of the radicular veins.

From Morris's treatise on anatomy.