THE CENTRAL NERVOUS SYSTEM. 339 
it is laterocaudad. The nerves in the sacral and caudal regions 
pass almost directly caudad to reach the intervertebral foramina 
and form thus a brush which surrounds the filum terminale 
(Fig. 136, C) and is called the cauda equina (Fig. 136, B). 
Each spinal nerve immediately after leaving the interverte- 
bral foramen divides into two branches, a dorsal or posterior 
branch or ramus (c), and a ventral or anterior ramus (2). 
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. 
