THE NERVOUS SYSTEM. 



91 



three portions, columns, or tracts, an anterior, lateral, and posterior. From 

 the groove between the interior and lateral columns spring the anterior 

 roots of the spinal nerves (B and c, 5); and just in front of the groove 

 between the lateral and posterior column arise the posterior roots of the 

 same (B, G) : a pair of roots on each side corresponding to each vertebra 

 (Fig. 317). 



White m after. The white matter of the cord is made up of medullated 

 nerve fibres, of various sizes, arranged longitudinally around the cord under 



FIG. 316. Different views of a portion of the spinal cord from the cervical region, with the roots 

 of the nerves (slightly enlarged). In A, the anterior surface of the specimen is shown; the anterior 

 nerve-root of its right side being divided; in B, a view of the right side is given; in c, the upper sur- 

 face is shown: in D, the nerve-roots and ganglion are shown from below. 1. The anterior median fis- 

 sure; 2. posterior median fissure; 3, anterior lateral depression, over which the anterior nerve-roots 

 are seen to spread; 4, posterior lateral groove, into which the posterior roots are seen to sink; 5, 

 anterior roots passing the ganglion; 5'. in A, the anterior root divided; 6, the posterior roots, the 

 fibres of which pass into the ganglion 6' ; 7, the united or compound nerve ; 7', the posterior primary 

 branch, seen in A and D to be derived in part from the anterior and in part from the posterior root. 

 (Allen Thomson.) 



the pia mater and passing in to support the individual fibres in the delicate 

 connective tissue or neuroglia made up of a very fine reticulum, with both 

 small cells almost filled up by nuclei and stellate, branching corpuscles. 



Size. The general rule respecting the size of different parts of the 

 cord appears to be, that the size of each part bears a direct proportion 

 to the size and number of nerve-roots given off from itself, and has but 

 little relation to the size or number of those given off below it. Thus 

 the cord is very large in the middle and lower part of its cervical portion, 

 whence arise the large nerve-roots for the formation of the brachial plex- 

 uses and the supply of the upper extremities, and again enlarges at the low- 

 est part of its dorsal portion and the upper part of its lumbar, at the origins 



