The spinal nerves are arranged in pairs, the nerves of each pair being symmetrical in their attachment to either side of their respective segment of the spinal cord, and, in general, symmetrical in their course and distribution.

There are usually thirty-one pairs of functional spinal nerves. For purposes of description these are topographically separated into eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pairs of lumbar, five pairs of sacral, and one pair of coccygeal nerves. Occasionally the coccygeal or thirty-first pair is practically wanting, while, on the other hand, there may be frequently found small filaments representing one or even two additional pairs of coccygeal nerves below the thirty-first pair. These rudimentary coccygeal nerves are probably not functional. They never pass outside the vertebral canal, and often even remain within the tubular portion of the filum terminale. There sometimes occurs an increase in the number of vertebrae in the vertebral column and in such cases there is always a corresponding increase in the number of the spinal nerves.

Origin and attachment

Each spinal nerve (unlike the cranial nerves) is attached to the spinal cord by two roots: a sensory or afferent dorsal root [radix posterior] and a motor or efferent ventral root [radix anterior]. Each dorsal root has interposed in its course an ovoid mass of nerve-cells, the spinal ganglion, and the nerve-fibers forming the root arise from the cells of this ganglion and are thus of peripheral origin. The fibers composing the ventral root, on the other hand, are of central origin; they arise from the large motor cells of the ventral horn of the grey column within the spinal cord.

Each dorsal root-fiber upon leaving its cell of origin pursues a short tortuous course within the spinal ganglion and ^hen undergoes a T-shaped bifurcation, one product of which passes toward the periphery, where it terminates for the collection of sensations and is known as the peripheral branch, or, since it conveys impulses toward the cell-body, the dendrite of the spinal

Ventral and Dohsal Views op Spinal Cord showing Manner of Attachment OF Dorsal and Ventral Roots.

ganglion neurone. It is more correct, however, to consider the T-fiber as a bifurcated axone. The other product of the bifurcation, the central branch, passes into the spinal cord and in its course toward the cord contributes to form the dorsal root proper.

The central branches, upon emerging from the spinal ganglia, form a single compact bundle at first, which passes through the dura mater of the spinal cord and then breaks up into a series, of root-filaments [fila radicularia]. These thread-like bundles of fibers spread out vertically in a fan-like manner and enter the cord in a direct linear series along its postero-lateral sulcus. The fibers of the ventral root emerge from the cord in a series of more finely divided root filaments, which, unlike the entering filaments of the dorsal root, are not arranged in direct linear series, but make their exit over a strip of the ventro-lateral aspect of the cord in some places as much as two millimetres wide.

As they enter the spinal cord the fibers of the dorsal roots undergo a Y-shaped division, both products of which course in the cord longitudinally, an ascending and a descending branch. The descending or caudal branches are shorter than the ascending, and soon enter and terminate about the cells within the grey column of the cord, forming either associational, commissural, or reflex connections, or about cells whose fibers form cerebellar connections. The ascending or cephalic branches are either short, intermediate, or long. The short and intermediate branches are similar in function to the descending branches, save that they become associated with the grey substance of segments of the cord above rather than below the level of their entrance. The long branches convey impulses destined for the structures of the brain, and pass upward in the fasciculus gracilis or fasciculus cuneatus of the cord, and terminate in the nuclei of these fasciculi in the meduUa oblongata.

Aberrant spinal ganglia

In serial sections on either side of the spinal ganglion of a nerve there may often be found outlying cells either scattered or in groups of sufficient size to be called small gangUa. Such are more often found in the dorsal roots of the lumbar and sacral nerves. These cells are nothing more than spinal ganghon-cells displaced in the growth processes, and have the same nature and function as those in the ganghon. In some animals occasional cells very rarely have been found in the outer portion of the ventral root. These probably represent afferent fibers which enter the cord by way of the ventral root. Likewise, especially in the birds and amphibia, it has been shown that occasional efferent fibers may pass from the grey substance of the cord to the periphery by way of the dorsal instead of the ventral root.

Relative size of the roots

The sensory or dorsal root is larger than the ventral root, indicating that the sensory area to be supplied is greater and perhaps more abundantly innervated than the area requiring motor fibers.

It has been shown that in the entire thirty-one spinal nerves of one side of the body of man the dorsal root-fibers number 653,627, while all the corresponding ventral roots contain but 233,700 fibers, a ratio of 3.2 : 1. (Ingbert.) In the increase in the size of the nerves for the supply of the limbs the gain of dorsal root or sensory fibers is far greater than the gain of ventral root-fibers. The first cervical or the sub-occipital nerve is always an exception to the rule; its dorsal root is always smaller than its ventral, and in rare cases may be rudimentary or entirely absent. The spinal ganghon and, therefore, the sensory root of the coccygeal nerve, is also quite frequently absent.

The dorsal and ventral root-fibers of each spinal nerve proceed outward from their segment of attachment to the spinal cord, pierce the pia mater and arachnoid, collect to form their respective roots, and pass into their respective intervertebral foramina. On the immediate peripheral side of the spinal ganglion the two roots blend, giving origin to the thus mixed nerve-trunk. As the trunk, the sensory and motor fibers make their exit from the vertebral canal through the intervertebral foramen.

Relation to the meninges

The root filaments of each nerve receive connective-tissue support from the pia mater and arachnoid in passing through them. In the sub-arachnoid cavity they become assembled into their respective nerve-roots; and the roots, closely approaching each other, pass into the dura mater, from which they receive separate sheaths at first, but at the peripheral side of the ganglion these sheaths blend into one, which, with the subsequent blending of the roots, becomes the sheath or epineurium of the nerve trunk. By means of the sheaths derived from the meninges, especially the dura, the nerve-roots and the trunk are attached to the periosteum of the margins of the intervertebral foramina and thus are enabled to give some lateral support to the spinal cord in the upper portion of the canal.

The majority of the spinal ganglia lie in the intervertebral foramina, closely ensheathed, and thus outside the actual sac or cavity of the dura mater. The ganglia of the last lumbar and first four sacral nerves he inside the vertebral canal, but since the sheath derived from the dura mater closely adheres to them, they are still outside the sac of the dura mater. The ganglia of the last sacral and of the coccygeal nerves (when present) lie in tubular extensions of the sub-dural cavity, and thus not only within the vertebral canal, but actually within the sac of the dura mater. The trunk of the first cervical nerve is assembled within the sac of the dura mater, and, therefore, the spinal ganghon of this nerve, when present, may he within the sac.

Course and direction of emergence

Invested with the connective-tissue sheath derived from the meninges, each thoracic, lumbar and sacral nerve emerges from the vertebral canal through the intervertebral foramen below the corresponding vertebra, and all the nerves are in relation with the spinal rami of the arteries and veins associated with the blood supply of the given localities of the spinal cord.

The first cervical nerve does not pass outward in an intervertebral foramen proper, but between the occipital bone and the posterior arch of the atlas and beneath the vertebral artery. Thus the eighth or last cervical nerve emerges between the seventh cervical and the first thoracic vertebra.

The first and second pairs of cervical nerves pass out of the vertebral canal almost at right angles to the levels of their attachment to the spinal cord. During the early periods of development the level of exit of each pair of spinal nerves is opposite the level of its attachment to the cord, but, owing to the fact that in the later periods the vertebral column grows more rapidly than the cord and increases considerably in length after the cord has practically ceased growing, all the spinal nerves, with the exception of the first two, pass downward as well as outward. The obhquity of their course from the level of attachment to the level of exit increases progressively from above downward, and, as the cord ends at the level of the first or second lumbar vertebra, the roots of the lower lumbar and of the sacral nerves pass at first vertically downward within the dura mater, and form aroimd the filum terminale a tapering sheaf of nerve-roots, the Cauda equina (horse's tail).

Topography of attachment

The relations between the levels of attachment of the spinal nerves to the cord and the spinous processes of the vertebrae situated opposite these levels have been investigated by Nuhn and by Reid. The following table compiled by Reid gives the extreme limits of attachment as observed in six subjects.

Table of Topography of Attachment of Spinal Nerves to the Spinal Cord. (Reid.)

(A) signifies the highest level at which the root filaments of a given nerve are attached to the cord, and (B) the lowest level observed. For example, the root filaments of the sixth thoracic nerve may be attached as high as the lower border of the spinous process of the second thoracic vertebra, or some may be attached as low as the upper border of the spinous process of the fifth thoracic vertebra, but in a given subject they do not necessarily extend either as high or as low as either of the levels indicated.

Nerves

Second cervical (A) A little above the posterior arch of atlas.

(B) Midway between posterior arch of atlas and spine of epistropheus.

Third cervical (A) A little below posterior arch of atlas.

(B) Junction of upper two-thirds and lower third of spine of epistropheus.

Fourth cervical (A) Just below upper border of spine of epistropheus.

(B) Middle of spine of third cervical vertebra.

Fifth cervical (A) Just below lower border of spine of epistropheus.

(B) Just below lower border of spine of fourth cervical vertebra.

Sixth cervical (A) Lower border of spine of third cervical vertebra.

(B) Lower border of spine of fifth cervical vertebra.

Seventh cervical (A) Just below upper border of spine of fourth cervical vertebra.

(B) Just above lower border of spine of sixth cervical vertebra.

Eighth cervical (A) Upper border of spine of fifth cervical vertebra.

(B) Upper border of spine of seventh cervical vertebra.

First thoracic (A) Midway between spines of fifth cervical and sixth cervical vertebra.

(B) Junction of upper two-thirds and lower third of interval between seventh cervical and first thoracic vertebra.

Second thoracic (A) Lower border of spine of sixth cervical vertebra.

(B) Just above lower border of spine of first thoracic vertebra.

Third thoracic (A) Just above middle of spine of seventh cervical vertebra.

(B) Lower border of spine of second thoracic vertebra.

Fourth thoracic (A) Just below upper border of spine of first thoracic vertebra.

(B) Junction of upper third and lower two-thirds of spine of third thoracic vertebra.

Fifth thoracic (A) Upper border of spine of second thoracic vertebra.

(B) Junction of upper quarter and lower three-quarters of spine of fourth thoracic vertebra.

Sixth thoracic (A) Lower border of spine of second thoracic vertebra.

(B) Just below upper border of spine of fifth thoracic vertebra.

Seventh thoracic  (A) Junction of upper third and lower two-thirds of spine of fourth thoracic vertebra.

(B) Just above lower border of spine of fifth thoracic vertebra.

Eighth thoracic (A) Junction of upper two-thirds and lower third of interval between spines of fourth thoracic and fifth thoracic vertebra.

(B) Junction of upper quarter and lower three-quarters of spine of sixth thoracic vertebra.

Ninth thoracic (A) Midway between spines of fifth thoracic and sixth thoracic vertebra.

(B) Upper border of spine of seventh thoracic vertebra.

Tenth thoracic (A) Midway between "spines of sixth thoracic and seventh thoracic vertebra.

(B) Middle of the spine of eighth thoracic vertebra.

Eleventh thoracic (A) Junction of upper quarter and lower three-quarters of spine of seventh thoracic vertebra.

(B) Just above spine of ninth thoracic vertebra.

Twelfth thoracic (A) Junction of upper quarter and lower three-quarters of spine of eighth thoracic vertebra.

(B) Just below spine of ninth thoracic vertebra.

First lumbar (A) Midway between spines of eighth thoracic and ninth thoracic vertebrae.

(B) Lower border of spine of tenth thoracic vertebra.

Second lumbar (A) Middle of spine of ninth thoracic vertebra.

(B) Junction of upper third and lower two-thirds of spine of eleventh thoracic vertebra.

Third lumbar (A) Middle of spine of tenth thoracic vertebra.

(B) Just below spine of eleventh thoracic vertebra.

Fourth lumbar (A) Just below spine of tenth thoracic vertebra.

(B) Junction of upper quarter and lower three-quarters of spine of twelfth thoracic vertebra.

Fifth lumbar (A) Junction of upper third and lower two-thirds of spine of eleventh thoracic vertebra.

(B) Middle of spine of twelfth thoracic vertebra.

First sacral (A) Just above lower border of spine of eleventh thoracic vertebra.

Fifth sacral (B) Lower border of spine of first lumbar vertebra.

Coccygeal (A) Lower border of spine of first lumbar vertebra.

(B) Just below upper border of spine of second lumbar vertebra.

Relative size of the nerves

The size of the different spinal nerves varies greatly. Just as the spinal cord shows marked enlargements in the cervical and lumbar regions necessitated by the greater amount of innervation required of these regions for the structures of the upper and lower limbs, so the nerves attached to these regions are considerably larger than elsewhere.

The smaller nerves are found at the two extremities of the cord and in the mid-thoracic region. The smallest nerve is the coccygeal, and the next in order of size are the lower sacral and the first two or three cervical nerves. The largest nerves are those which contribute most to the great nerve trunks for the innervation of the skin and muscles of the limbs: the lower cervical and first thoracic for the upper limbs and the lower lumbar and first sacral for the lower Umbs. The nerves gradually increase in the series in passing from the smaller toward the larger.

Diagrams Illustrating the Origin and Distribution of a Typical Spinal Nerve. A, in thoracic region; B, in region of a limb (highly schematic).

The primary divisions of the nerve-trunk

A typical spinal nerve (middle thoracic, for example), just as it emerges from the intervertebral foramen, divides into four branches: the two large primary divisions; viz., the posterior primary division [ramus posterior] and the anterior primary division [ramus anterior]; third, the small ramus communicans, by which it is connected with the sympathetic; and fourth, the smaller, ramus meningeus {recurrent branch), which immediately turns centralward for the innervation of the membranes and vessels of the spinal cord.

In general, the posterior primary division passes dorsalward between the arches or transverse processes of the two adjacent vertebrae in relation with the anterior costo-transverse ligament, and then divides (with the exception of the first cervical, the fourth and fifth thoracic, and the coccygeal nerves) into a medial (internal) branch and a lateral (external) branch. The medial branch turns toward the spinous processes of the vertebrse, and supplies the bones and joints and the muscles about them, and may or may not supply the skin overlying them. The lateral branch turns dorsalward and also supplies the adjacent muscles and bones, and, if the medial branch has not supplied the overlying skin, it also terminates in cutaneous twigs.

In the upper half of the spinal nerves the medial branches supply the skin; in the lower half, it is the lateral branches which do so. Both branches of almost aU the posterior divisions, especially those of the lower nerves, show a tendency to run caudalward and thus are distributed to muscles and skin below the levels of their respective intervertebral foramina. They never supply the muscles of the limbs, though their cutaneous distribution extends upon the buttock, the shoulder, and the skin of the back of the head as far upward as the vertex. The posterior primary divisions, with the exception of those of the first three cervical nerves, are much smaller than the anterior primary divisions.

As their mixed function suggests, the posterior primary divisions contain both nerve-fibers from the ventral roots and peripheral processes of the spinal ganghon-cells. If the nerve-trunk on the immediate peripheral side of the spinal ganghon be teased, bimdles of ventral root-fibers may be seen crossing the trunk obliquely to enter the posterior division, and fibers from the spinal ganghon may be also traced into it. Also a few sympathetic fibers, derived chiefly by way of the ramus communicans, are known to course in it for distribution in the walls of the blood-vessels, etc., of the area it suppUes.

The anterior primary divisions run lateralward and ventralward. With the exception of the first two cervical nerves, which contribute the hypoglossal loop, they are larger than the posterior primary divisions, and appear as direct continuations of the nerve-trunks. Only in case of most of the thoracic nerves do thej^ remain independent in their course. In these they run lateralward and ventralward in the body-wall. In general, these divisions supply the lateral and ventral

Diagram illustrating the origin of the component nerve-fibers of the primary divisions of a typical spinal nerve.

parts of the body, the limbs, and the perineum. In the cervical, lumbar, and sacral regions they lose their anatomical identity by dividing, subdividing, and anastomosing with each other so as to give rise to the three great spinal plexuses of the body: the cervical, the brachial, and the lumbo-sacral plexuses. The majority of the thoracic nerves retain the typical or primitive character in both their anterior and posterior primary divisions. In them the anterior division (intercostal nerve) divides into a lateral or dorsal and an anterior or ventral branch, both of which subdivide. The lateral branch is chiefly cutaneous; it pierces the superficial muscles and, in the subcutaneous connective tissue, divides into a smaller posterior and a larger anterior ramus, which respectively supply the skin of the sides and the lateral part of the ventral surface of the body. The anterior branch continues ventralward iii the body-wall, giving off twigs along its course to the adjacent muscles and bones, and, as it approaches the ventral mid-fine of the body, it turns sharply lateralward and sends rami medialward and lateralward to supply the skin of the ventral aspect of the bodjr. In the region of the limbs the typical arrangement is interfered with in that what corresponds to the lateral and anterior branches of the division are carried out into the limbs for the skin and muscles there, instead of supplying the lateral and ventral parts of the body-wall.

Nerve-fibers arising in the spinal ganglion and fibers from the ventral root pass directly from the nerve-trunk into the anterior primary division of the spinal nerve. This division also receives sympathetic nerve-fibers by way of the ramus communicans. These latter accompany the division and are distributed to their allotted elements in the territory it supplies.

Table Giving the Approximate Areas of Distribution of the Different Spinal Nerves with a Diagram showing Their Respective Levels of Exit from the Vertebral Column. (Arranged by Dr. Gowers.)

The rami communicantes are small, short, thread-like branches by which the nerve-trunks are connected with the nearest ganglion of the vertically running gangliated cord of the sympathetic (sympathetic trunk). The trunk or anterior primary division of every spinal nerve has at least one of these; most of the nerves have two, and sometimes there are three. The nerves of the cervical region usually have but one, and this is composed largely of sympathetic fibers (grey ramus). Where there are two, one usually contains medullated fibers, chiefly from the ventral root, sufficient to give it a whiter appearance (white ramus).

In the upper cervical and in the sacral regions one sympathetic ganglion may be connected with two or more spinal nerves, and sometimes one nerve is connected with two ganglia. The rami communicantes of the spinal nerves are equivalent to the communicating rami connecting certain of the cranial nerves with the sympathetic system (trigeminus, glosso-pharyngeus, vagus) . The medullated fibers of the rami and, therefore, the white rami consist chiefly of fibers from the spinal nerves, viz., fibers from the spinal ganglion-cells which enter and course to their distribution through branches of the sympathetic nerves, visceral afferent fibers, and fibers from the ventral roots of the spinal nerves which terminate in the sympathetic ganglia, visceral efferent (preganghonic) fibers. Thus the white rami have been termed the visceral divisions of the spinal nerves. The grey rami consist chiefly of sympathetic fibers, most of which are non-meduUated or partially medullated, and which course to their distribution by way of the spinal nerves. Some of the sympathetic fibers terminate in the spinal ganghon, afferent sympathetic fibers. The usual absence of white rami communicantes from the cervical nerves is explained on the grounds: (1) that probably relatively few efferent visceral fibers are given to the sympathetic from this region of the cord; (2) that many of the visceral efferent fibers which do arise from this region of the cord probably join the rootlets of the spinal accessory nerve and pass to the sympathetic system through the trunk of this nerve, and through the vagus with which it anastomoses; and (3) that such of these fibers as are given off from the lower segments of the cervical region, descend the cord and pass out by way of the upper thoracic nerves which give very evident white rami to the sympathetic.

The meningeal or recurrent branch is very small and variable, and is often difficult to find in ordinary dissections. It is given off from the nerve-trunk just before its anterior and posterior primary divisions are formed. It consists of a few peripheral branches of spinal ganglion-cells (sensory fibers) which leave the nerve-trunk and re-enter the vertebral canal for the sensory innervation of the meninges, and which are joined by a twig from the grey ramus or directly from the nearest sympathetic ganglion (vaso-motor fibers). There is considerable evidence, both physiological and anatomical, obtained chiefly from the animals, which shows that at times certain of the peripheral spinal ganghon or sensory fibers may turn backward in the nerve-trunk and pass to the meninges within the ventral root instead of contributing to a recurrent branch. The occurrence of such fibers in the ventral root explains the physiological phenomenon known as 'recurrent sensibility.' Likewise, sympathetic fibers entering the trunk through the grey ramus may pass to the meninges by way of the ventral root, and at times the recurrent branch is probably absent altogether, its place being taken entirely by the meningeal fibers passing in the ventral root.

Areas of distribution of the spinal nerves

Both the posterior and anterior primary divisions divide and subdivide repeatedly, and their component fibers are distributed to areas of the body more or less constant for the nerves of each pair, but the distribution of the different nerves is very variable. Corresponding to their attachment, each to a given segment of the spinal cord, the nerves have primarily a segmental distribution, but, owing to the developmental changes and displacement of parts during the growth of the body, the segmental distribution becomes greatly obscured and in some nerves practically obliterated. Naturally it is more retained by the nerves supplying the trunk than by those contributing to the innervation of the limbs and head, and the areas supplied by the posterior primary divisions are less disturbed than those supplied by the anterior. The segmental areas of cutaneous distribution of the posterior divisions are more evident than the areas of muscle supplied by these divisions, from the fact that the segmental myotomes from which the dorsal muscles arise fuse together and overlap each other considerably during development. No nerve has a definitely prescribed area of distribution, cutaneous or muscular, for its area is always considerably overlapped by the areas of the nerves adjacent to it. The mid-thoracic nerves more nearly supply a definitely prescribed belt of the body.

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