PERIPHERAL NERVOUS SYSTEM 



173 



lobes, the corpora quadrigemini, only the anterior pair of which are connected 

 with the optic nerves, the posterior pair being a centre connected with audition. 

 The aqueduct is small in diameter. Behind the mid-brain the roof of the aque- 

 duct is epithelial and is called the velum meduUare anterius (fig. 181, »). 

 The cerebellum is large, and its surface is increased by an extensive folding, the 

 gray matter being on the surface, so that in longitudinal section (figs. 167, 181), 

 it presents a strikingly dendritic appearance, the so-called arbor vit«. The size 

 is largely due to the great development of the cerebellar hemispheres, the vermis 

 and the flocculi (the latter usually subdivided into flocculus and paraflccculus) 

 being less conspicuous. 



Fig. 182. Fig. 183. 



Fig. 182. — Brain of Chrysothryx sciureus, after Weber. /, frontal lobe; i, inter- 

 parietal fesure; 0, occipital lobe; p, parietal lobe; s, Sylvian fissure; /, temporal lobe, Is, 

 sulcus temp>orali5. 



- — Fig. 183. — Brain of Mani javanica, after Weber. cA, cerebellar hemisphere; h, 

 hippocampal lobe; 0, olfactory lobe; ps, presylvian fissure; s, Sylvian fissure; ss, sulcus 

 sagittalis; v, vermis; //.optic nerve. 



THE PERIPHERAL NJ 



^OUS 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. 153). By unequal 

 growth each crest soon develops a series of metameric enlargements, 

 the portions of the crest between these gradually disappearing, while 

 the enlargements form the ganglia of the dorsal roots of the nerves. 

 Each of the cells (neuroblasts), of which each ganglion is comp>osed, 

 like those of the cord, sends out processes, one of which grows medi- 

 ally and enters the cord in the region of the posterior column, where it 



