si'i.\.\L \i-:h'\'i-:s 901 



vascular systems, while some serve as secretory fibres to (he glands. (6) Cerebro 

 spinal nervc-lil)res are characteri/cd by well-developed medullary sheaths, making 

 tlie nerve-- appear a> \\liite .strands; most of the sympathet ic fibres are nori-inedul- 

 lated, some are partially mediillated. but none possess as thick medullary sheaths as 

 those of the cerebro-spitial nerves. Thus sympathetic nerves appear as grey strands. 

 The cerebro-spinal nerves. There are forty- three pairs ,,f ceivbru-spinal nerves, 

 of which thirty-one, pairs are attached to the spinal cord (spinal nerves) and twelve 

 pairs to the encephalon (cranial nerves). The spinal nerves are the more primitive 

 and retain the typical character, i.e., each is attached to the spinal cord by two 

 roots, a dorsal or sensory ganglionated root, and a ventral, which is motor, and thus 

 not ganglionated. .Most of the cranial nerves have only one root , which in some cases 

 corresponds to a dorsal root and therefore has a ganglion, and in other cases corre- 

 sponds, physiologically at least, to a ventral root of a spinal nerve. Among other 

 differences, the fibres of the first cranial nerve, for example, do not collect to form a 

 distinct nerve-trunk. On account, therefore, of their more typical characters, it is 

 convenient to consider the spinal nerves first. 



THE SPINAL NERVES 



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 topographic- 

 ally 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 addi- 

 tional pairs of coccygeal nerves below the thirty-first pair. These rudimentary 

 coccygeal nerves are probably not functional. They never pass outside the verte- 

 bral canal, and often even remain within the tubular portion of the filum terminate. 

 There sometimes occurs an increase in the number of vertebra 1 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 is attached to the spinal cord by 

 two roots: a sensory or afferent dorsal root and a motor or efferent ventral root. 

 Each dorsal root has interposed in its course an ovoid mass of nerve-cells, the spinal 

 ganglion, and the nerve-fibres forming the root arise from the cells of this ganglion 

 and are thus of peripheral origin. The fibres 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-fibre upon leaving its 

 cell of origin pursues a short tortuous course within the spinal ganglion and then 

 undergoes a T-shaped bifurcation, one product of which passes towards the periphery, 

 where it terminates for the collection of sensations and is known as the periphtml 

 branch, or, since it conveys impulses towards the cell-body, the dendrile of the spinal 

 ganglion neurone. The other product of the bifurcation, the central branch, passes 

 into the spinal cord and in its course towards the cord contributes to form the dorsal 

 root proper. 



The ccMitral branches, upon emerging from the spinal ganglia, form a single com- 

 pact 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 fibres spread out vertically in a fan-like manner and enter the cord in a direct 

 linear series along its postero-lateral sulcus. The fibres of the ventral root em. 

 from the cord in a series of more finely divided root filaments, which, unlike the enter- 

 ing filaments of the dorsal root, are not arranged in direct linear series, but make their 

 exit over a strip of the vent ro-lat era! aspect of the cord in some places as much as 

 two millimetre-; wide. 



As they enter the spinal cord the fibres of the dorsal runt- undergo a Y-sh:<i>ed divi-ion, 

 both products of which course in the curd longitudinally, .-in a-rrniiini: :ind a de-cendin<: branch. 

 The descending or caudad branches arc -hortrr than the ascending, and soon enter and terminate 

 alioiit the cells within the grey column of (lie cord, forming either a>.soi iational. commissural. or 

 reflex connections, or about cells whose filires form cercliellar connections. The ascending 

 or cephalad branches are cither short, intermediate, or long. The short and intermediate 



