78 THE NERVOUS SYSTEM OF THE FROG. 
v. The anterior fissure of the brain: a median longi- 
tudinal groove on the ventral surface of the hinder 
part of the brain: continuous with a similar 
groove on the ventral surface of the spinal cord. 
II. The spinal cord. 
A somewhat flattened band presenting brachial and lumbar 
enlargements opposite the points of origin of the nerves for the 
fore and hind limbs respectively, and slightly constricted between 
these two points. About the level of the sixth or seventh ver- 
tebra the cord narrows rapidly to form a fine thread, the filum 
terminale, which extends back into the canal of the urostyle. 
Besides the anterior or ventral fissure the cord presents a 
dorsal fissure, a median longitudinal groove starting from the 
hinder angle of the fourth ventricle and extending to the com- 
mencement of the filum terminale. 
The tubular character of the spinal cord is best seen on 
making transverse sections of it. These will be described 
further on in the present chapter. 
B. The Peripheral Nervous System. — 
I. The Spinal Nerves: Ten pairs of nerves arise from the 
sides of the spinal cord; each nerve arising by two roots, 
a ventral or “anterior,” and a dorsal or ‘ posterior,” which unite 
together at their point of exit from the vertebral canal through 
the intervertebral foramen: just before their union the pos- 
terior root bears a ganglionic swelling. 
Within the vertebral canal the roots of the anterior spinal 
nerves run nearly transversely outwards, so as to leave the 
canal opposite their points of origin from the spinal cord. The 
roots of the middle and posterior nerves, owing to the vertebral 
column being of greater length than the part of the cord 
belonging to it, pass obliquely backwards to their points of exit : 
and in the case of the hindmost nerves, the roots run backwards 
within the vertebral canal some distance before reaching their 
foramina of exit: the bundle formed by these roots, together 
with the filum terminale, is spoken of as the cauda equina. 
a. The spinal nerves within the vertebral canal: 
To expose the nerves either cut away with scissors the neural 
