758 



THE NERVOUS SYSTEM 



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 

 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 

 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 memnges, 

 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 medul- 

 laris, 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 



FIQ. 565. DORSAL VIEW OF PORTION OF SPINAL CORD IN POSITION IN VERTEBRAL CANAL. 



A*- ARACHKOIDEA SPIN A US 

 DURA MATER SPIN A LIS - ~afoj ^|fc 



LOWER CERVICAL 

 REGION 



, SPINAL NERVE 



THORA CIC REGION 



spinal cord, and forms the non-nervous, slender, thread-like terminal, the filum 

 terminate. This becomes continuous with the dura mater at its lower extremity. 



In the early foetus the spinal nerves pass from their attachment to the spinal 

 cord outwards through the intervertebral foramina at right angles to the long axis of 

 the cord, but, owing to the fact that the vertebral column increases considerably in 

 length after the spinal cord has practically ceased growing, the nerve-roots become 

 drawn caudad from their points of attachment, and, as is necessarily the case, their 

 respective foramina are displaced progressively downwards as the termination of the 

 cord is approached, until finally the roots of the lumbar and sacral nerves extend 

 downwards as a brush of parallel bundles considerably below the levels at which they 

 are attached. This brush of nerve-roots is the cawla cquina. 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 envelopes the cauda equina, undergoing a 

 slightly bulbous, conical termination. 



The enlargements. Wherever there is a greater mass of tissue to be 

 innervated, the region of the nervous system supplying such must of necessity pos- 

 sess a greater number of neurones. Therefore, the regions of the spinal cord asso- 

 ciated 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 inner- 

 vated. Thus in the lower cervical region the spinal cord becomes broadened into 

 the cervical enlargement, and likewise in the lumbar region occurs the lumbar 



