THE NERVOUS SYSTEM 109 



nerves, and with them escape through the posterior lacer- 

 ated foramen. Trace them to the muscles of the neck. 



The twelfth nerves (hypoglossal) leave the medulla be- 

 hind the first ten cranial nerves, escape from the skull 

 through the hypoglossal canals, and may be traced easily 

 to the ventral part of the tongue. 



Exercise XXVIII. Draw a ventral view of the brain, showing the 

 origins of the cranial nerves. 



Using a very sharp razor cut the brain in two along a 

 plane parallel to, and a millimeter or two at the left of, 

 the longitudinal fissure. Place the larger piece in water 

 and cautiously dissect away the remnants of the left half 

 of the brain, exposing the structures described below. 



The corpus callosum is the thick commissure connecting 

 the cerebral hemispheres. Posteriorly it takes the form of 

 an oval mass (splenium) ; anteriorly it bends downward 

 (genu), forming the rostrum. This section exposes a re- 

 gion of the brain not observed on the dorsal side of the 

 organ — the thalamencephalon, or diencephalon. The dien- 

 cephalon of the dogfish, frog and other lower vertebrates is 

 visible dorsally, but in mammals the backward extension 

 of the cerebral hemispheres, which is in general coincident 

 with the development of higher intelligence, conceals the 

 diencephalon. 



The third ventricle is the cavity within the diencephalon. 

 The fornix is dorsal to this ventricle. From its union, 

 posteriorly, with the corpus callosum, the fornix runs for- 

 ward and downward anterior to the third ventricle. Ob- 

 serve the anterior commissure at the lower end of the fornix. 

 The lamina terminalis is the anterior boundary of the third 

 ventricle. The conspicuous intermediate mass, or middle 

 commissure, extends from one lateral wall to the dien- 

 cephalon to the other through the third ventricle. The m- 



