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.