172 THE HUMAN BODY. 



uniting the right and left halves of the cerebellum; the 

 medulla oblongata and crura contain mainly longitudinal 

 fibres, but there are many transverse. 



Fig. 76 represents a vertical transverse section of the brain 

 taken through the forepart of the corpus callosum (CcF) 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 ventricles, VI, with a gray mass, the 

 corpus striatum lying beneath and on the outer side of each. 

 If the section had been taken a little farther back the optic 

 tlialami would have been found reaching the floor of each ven- 

 tricle. Like the optic thalamus, to the front of and partly to 

 the outer side of which it lies, the corpus striatum is mainly 

 composed of gray nerve matter. It is, however, divided in 

 its posterior region into an inner and outer portion by a well 

 marked band of white substance, consisting of nerve fibres, 

 passing through on the way to or from the surface of the 

 cerebral hemispheres: this band is the internal capsule. 



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. 77. The first pair, or olfactory nerves, spring from 

 the under sides of the olfactory lobes, 1, 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 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 com- 

 missure (seen in section at z, in Fig. 75), from which an optic 

 nerve proceeds to each eyeball. Behind the optic commis- 

 sure is seen the conical stalk of the pituitary body or hy- 

 pophysis cerebri (pt in Fig. 75), 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 muscles 

 which move the eyeball and also to that which lifts the upper 

 eyelid. The four-sided space bounded by the optic tracu; 



