964 



THE NERVOUS SYSTEM 



II. THE SPINAL NERVES 



The spinal nerves are arranged in pairs, the nerves of each pair being symmet- 

 rical 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, jive pairs of lumbar, jive pairs of sacral, and one pair of coccygeal 

 nerves. Occasionally the coccygeal or thirtj^-first pair is practically wanting, 

 while, on the other hand, there may be frequently found small filaments represent- 

 ing 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 correspond- 

 ing 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-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 

 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 



Fig. 746. — Ventral and Dorsal Views of Spinal Cord showing Manner of Attachment 



OF Dorsal and Ventral Roots. 



Antero-lateral sulcus (line of ventral roots) 

 /Anterior median fissure 



Posterior median sulcus 

 Posterior in- 

 /•termediate 

 sulcus 



/] 



Postero- 

 .^ _ lateral 

 'i'f^ sulcus 

 (line of 

 i • '^'^ dorsal 

 roots) 



V 



ganglion neurone. It is more correct, however, to consider the T-fibre 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 fir.st, wliich 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 emerge from the cord in a series of more finely divided root fila- 

 ments, which, unlike the entering filaments of the dorsal root, are not arranged in direct linear 

 series, but make tlieir exit over a strip of the ventro-lateral aspect of the cord in some places as 

 much aa two millimetres wide. 



As they enter the spinal cord the fibres 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 



