SPINAL NERVES. 161 
The cerebellum is divided into a median vermis and a pair of lateral portions, 
each consisting of a large cerebellar hemisphere, ventral (morphologically lateral) 
to which is a flocculus (fig. 162), homologous to that of the sauropsida. The 
surface of the hemispheres is convoluted and this results in the arrangement of 
the white and gray matter in such a way that they have a markedly dendritic ap- 
pearance (arbor vita, fig. 152) when seen in longitudinal section. The pons, 
‘characteristic of the mammalian brain, has already been mentioned (p. 150). 
THE PERIPHERAL NERVOUS SYSTEM. 
The Spinal Nerves. 
The spinal nerves are metameric structures, connected with the 
spinal cord by two separate portions or roots which differ greatly from 
each other in development, structure and function. At the time of 
the closure of the neural tube a band of cells occurs on either side of the 
neural plate at the junction of neural and epidermal areas. With the 
closure of the tube these form two bands, the neural crests, one on 
either side of the dorsal surface of the cord (fig. 144). By unequal 
growth each crest soon develops a series of metameric enlargements, the 
portions of the crest between these gradually disappearing, while the en- 
largements form the ganglia of the dorsal roots of the nerves. Each of 
its cells, like those of the cord, sends out processes, one of which grows 
medially and enters the cord in the region of the posterior cornu, while 
the other extends peripherally to the skin or viscera, these processes 
constituting the dorsal root of the nerve, the ganglion forming an 
enlargement upon it, near its connection with the cord. The other or 
ventral root is formed by fibres which grow out in a similar way from 
cells in the ventral horn of the cord itself and leave it between the an- 
terior and lateral columns, to extend to the muscles, glands, etc. As the 
ganglion cells are inside the cord, there is no ganglion on the ventral 
root. Except in the cyclostomes the dorsal and ventral roots unite 
soon after leaving the cord, the combined trunk being a typical spinal 
nerve (figs. 145, 166). 
Physiologically the roots differ in that the dorsal roots are mainly 
composed of sensory fibres, while the ventral roots contain only motor 
fibres. That is, on stimulation of the parts to which they are distrib- 
uted the dorsal roots and their fibres carry nervous impulses to the cord 
—they are afferent—while the impulses in the ventral roots are carried 
in the opposite direction by efferent fibres. In their case stimulation 
arises in the central nervous system and the impulse is carried outward 
Il 
