330 GENERAL STRUCTURE AND FUNCTIONS 



by an anterior and posterior median fissure, into two lateral halves, 

 which still remain connected with each other by a central mass or 

 commissure. Its inner portions are occupied by gray matter, 

 which forms a continuous ganglionic chain, running from one ex- 

 tremity of the cord to the other. Its outer portions are composed 

 of white substance, the filaments of which run for the most part in 

 a longitudinal direction, connecting the different parts of the cord 

 with each other, and the cord itself with the ganglia of the brain. 



The spinal nerves are given off from the spinal cord at regular 

 intervals, and in symmetrical pairs; one pair to each successive 

 portion of the body. Their filaments are distributed to the integu- 

 ment and muscles of the corresponding regions. In serpents, where 

 locomotion is performed by simple, alternate, lateral movements 

 of the spinal column, the spinal cord and its nerves are of the 

 same size throughout. But in the other vertebrate classes, where 

 there exist special organs of locomotion, such as fore and hind 

 legs, wings, and the like, the spinal cord is increased in size at 

 the points where the nerves of these organs are given off; and the 

 nerves themselves, which supply the limbs, are larger than those 

 originating from other parts of the spinal cord. Thus, in the hu- 

 man subject (Fig. 126), the cervical nerves, which go to the arms, 

 and the sacral nerves, which are distributed to the legs, are larger 

 than the dorsal and lumbar nerves. They form, also, by frequent 

 inosculation, two remarkable plexuses, before entering their corre- 

 sponding limbs, viz., the bra- 



Flg> 127> chial plexus above, and the 



sacral plexus below. The 

 cord itself, moreover, pre- 

 sents two enlargements at 

 the point of origin of these 

 nerves, viz., the cervical en- 

 largement from which the 

 brachial nerves (4, 4) are 

 given off, and the lumbar 



Transverse Section of SPINAL CORD.-,&. Spinal enlargement from which the 



nerves of right and left sides, showing their two roots. sacra l nerVCS (o, 5) Originate. 



d. Origin of anterior root. . Origin of posterior root. . j 



c. Ganglion of posterior root. If the Spinal COrd be 6Xa- 



mined in transverse section 



(Fig. 127), it will be seen that the gray matter in its central portion 

 forms a double crescentic-shaped mass, with the concavity of the 

 crescents turned outward. These crescentic masses of gray matter, 



