THE CENTRAL NERVOUS SYSTEM. 339 



it is laterocauclad. The nerves in the sacral and caudal regions 

 pass almost directly caudad to reach the intervertebral foramina 

 and form tlius a brush which surrounds the fdum terminale 

 (Fig. 136, C) and is called the Cauda equina (Fig. 136, B). 



F^ach spinal nerve immediately after leaving the interverte- 

 bral foramen divides into two branches, a dorsal or posterior 

 branch or ramus (<r), and a ventral or anterior ramus {d). 

 The dorsal ramus is in each case small (except in the first and 

 second cervical nerves), and is distributed to the longitudinal 

 muscles and integument of the back. 



The ventral rami are larger and each is connected a short 

 distance beyond its origin with the sympathetic system by a 

 short ramus communicans or communicating branch. Each 

 is then distributed to the integument and muscles of the ventral 

 part of the body, including the limbs. 



The ventral rami which pass to the limbs are much larger 

 than the others. The ventral rami are further distinguished 

 from the dorsal by the fact that they frequently unite with one 

 another to form plexuses. 



The peripheral distribution of the spinal nerves is described 

 later. 



2. The Brain. Encephalon. — The brain is that portion of 

 the central nervous system that is included within the cranial 

 cavity. It is a direct continuation of the spinal cord, and 

 presents many of the same essential characters as the latter, 

 with great modifications in details. 



The structure of the brain can best be understood if it be 

 considered as a modified continuation of the spinal cord, and 

 the relation of the parts to the essential parts of the cord noted. 

 The spinal cord is a nearly straight tube, with a central cavity 

 and thick walls. The brain is likewise tubular, with the 

 cavities enlarged or subdivided in places, with the walls greatly 

 thickened, and with a number of bends and constrictions in the 

 tube. The relation of the structure of the brain to that of the 

 spinal cord is most easily perceived by an examination of the 

 brain of some lower vertebrate, as the frog or shark, and such 

 an examination should be made before proceeding to the study 

 of the more complicated brain of the cat. 



