310 VERTEBRATE ZOOLOGY 



just back of the pons, and the pyramidal tracts, a pair of narrow, 

 elongated bands which occupy the median area between them. 

 The delicate abducent nerves spring from the anterior end of the 

 pyramidal tracts, and the hypoglossal nerves arise each by several 

 roots from their posterior portion ; the trigeminus, facial, auditory, 

 glossopharyngeal, and vagus nerves spring from the lateral surface 

 of the myelencephalon. 



Exercise 32. Draw an outline sketch of the ventral surface of the 

 brain, showing accurately these structures, so far as observed. 



Study the internal structure of the brain. The brain, like the 

 spinal cord, is a hollow structure. Unlike the spinal cord, how- 

 ever, in which the cavity is a simple tubular canal, the cavities of 

 the brain form a series of complicated spaces which extend 

 throughout its five divisions and their outgrowths. The brain 

 has thus dorsal, ventral, and lateral walls, which surround these 

 spaces ; these walls vary much in thickness in different places, the 

 actual form of the brain being due largely to these variations. 



We shall first study a longitudinal median section of the brain. 

 With a large, sharp scalpel or a razor, cut the brain into two equal 

 halves by a sagittal incision. Observe the conspicuous, broad band 

 of white fibers which joins the two hemispheres ; this is the corpus 

 callosum. Its posterior end is called the splenium; its anterior 

 end, which is enlarged and bends slightly ventrally, is the genu ; 

 curving ventrally from about the middle of it is the fornix. In the 

 angle between the anterior half of the corpus callosum and the 

 fornix is a membrane called the septum pellucidum, which is 

 the vertical partition separating the two anterior ventricles of the 

 brain, one of which is present in each of the hemispheres. If 

 the brain has not been divided exactly in the sagittal plane, one 

 of these ventricles may have been cut into. The ventral portion 

 of the fornix, which is called the column of the fornix, terminates 

 in a small, round body, the anterior commissure, from which a thin 

 plate, the lamina terminalis, appearing in the section as a thin 

 line, extends ventrally to the optic chiasma; these three struc- 

 tures form the anterior wall of the third ventricle of the brain, 

 which lies in the diencephalon. 



