THE BRAIN. 207 



the fourth ventricle. The cortical area for the nerve is 

 in the lower part of the convolutions about the fissure of 

 Rolando.) 



DISSECTION. 



Separate the halves of the cerebrum and examine the upper surface of the 

 corpus callosum. 



With a long, sharp knife slice off the top of the hemispheres, about half an 

 inch above the corpus callosum. The oval section presenting is called the 

 centrum ovale minus. Its surface will show the divided vessels, and its mar- 

 gin the arrangement of the white and gray matter. The formation of the 

 convolutions and sulci should be examined. 



Remove a slice of the brain on a level with the corpus callosum, opening 

 into the lateral ventricles. This section is called the centrum ovale majus. 

 Or the brain matter can be torn from the fibres of the corpus callosum on one 

 side, showing its arched formation and the forceps major and minor. While 

 on the other side the brain can be cut away, opening into the lateral ventricle. 



Consult the section, Fig. 30, which shows the horns of that ventricle, 

 and expose them in the specimen in a similar manner on this one side. 



The Corpus Callosum. 



The form of the great central commissure which joins 

 the two halves of the cerebrum must be learned from trans- 

 verse and anteroposterior sections of the brain. See Figs. 

 27, 30, 38, 45. 



The corpus callosum is composed of fibres which con- 

 nect symmetrical parts of the opposite hemispheres, but 

 these fibres being restricted to the central portion of the 

 opposing surfaces of the hemispheres, they become crowded 

 together, more at the anterior and posterior extremities. 



On cross section of the corpus callosum, this bunching 

 together of the fibres is best seen and the result is to 

 produce a thickening at these points. The posterior thick- 

 ening is called the splenium and the anterior the genu or 

 knee, because here the fibres of the corpus turn downward 

 to the base of the brain. This bend constitutes the genu 

 (knee), and the lower descending limb of the corpus gradu- 



