1G8 THE HUMAN BODY. 



the scat of the soul. Behind it come the corpora quactH- 

 gemina, Lq, and above the fourth ventricle the cerebellum, 

 Cbl, showing the primary and secondary fissures on its 

 surface which give its section a branched appearance known 

 as the arbor vitce. Mo is the medulla oblongata, and P the 

 pons Varolii. The canalis centralis of the spinal cord is 

 represented leading back from the fourth ventricle. 



Fig. 68 represents a vertical transverse section of the 

 brain taken through the fore part of the corpus callosum 

 (CcP) and altogether in front of the third ventricle. It 

 shows the foldings of the cerebrum and its superficial layer 

 of gray substance; the anterior ends of the lateral ventri- 

 cles, VI, with a gray mass, the corpus striatum lying be- 

 neath and on the outer side of each. If the section had 

 been taken a little farther back the optic tUalami would 

 have been found reaching the floor of each ventricle. 



The Base of the Brain and the Cranial Nerves. 

 Twelve pairs of nerves .leave the skull by apertures in its 

 'base, and are known as the cranial nerves. Most of them 

 spring from the under side of the brain, and so they are best 

 studied in connection with the base of that organ, which is. 

 represented in Fig. 70. The first pair, or olfactory nerves, 

 spring from the under sides of the olfactory lobes, /, and 

 pass out through the roof of the nose. They are the nerves 

 of smell. The second pair, or optic nerves, II, spring from 

 the optic thalami and corpora quadrigemina and, under 

 the name of the optic tracts, run down to the base of the 

 brain where they appear passing around the crura cerebri 

 as represented in the figure. In the middle line the two 

 optic tracts unite to form the optic commissure (seen in 

 section at // in Fig. 69) from which an optic nerve pro- 

 ceeds to each eyeball. Behind the optic commissure is 

 seen the conical stalk of the pituitary body or hypophysis 

 cerebri (II in Fig. 69), and still further back a pair of 

 hemispherical masses, about the size of split peas, known as 

 the corpora albicantia. 



All the remaining cranial nerves arise from the hind- 

 brain. The third pair (motor es oculi) arise from the front 

 of the pons Varolii, and are distributed to most of the 



