THE NERVOUS SYSTEM 219 



mamillaria. The posterior columns, or crura, descend from 

 the caudal border of the body, curving laterad into the median 

 cornu of the lateral ventricle, and gradually unite with the 

 hippocampus. The hippocampus is a thickened projecting fold 

 of the wall of the median cornu of the lateral ventricle (Fig. 

 107). The free lateral margin of the crus of the fornix is the 

 fimbria, or tcenia hippocampi. Adjacent to the median line 

 in either hemisphere, a thick lamina of matter, the septum 

 pellucidum, stretches from the cranial part of the fornix dorsad 

 to the callosum. The very narrow cavity formed by the ad- 

 herence of the margins of the septum of one hemisphere to 

 those of the septum in the other hemisphere is sometimes 

 called the fifth ventricle, or cavity of septum pellucidum. 

 Between each column of the fornix and the optic thalamus is 

 a cleft, the interventricular foramen leading from the third ven- 

 tricle, laterad of the septum pellucidum, into the lateral 

 ventricle. 



The three remaining commissures are known according to 

 their location as the anterior, middle, and posterior. The ante- 

 rior commissure (Figs. 106 and 109) perforates the corpora 

 striata, extending across the median line immediately craniad 

 of the columns of the fornix. It is about two millimeters in 

 diameter. The middle commissure, or massa intermedia, lies 

 between the thalami. It is sometimes called the soft or gray 

 commissure. It is nearly one centimeter in diameter and passes 

 through the third ventricle (Fig. 106). The posterior commis- 

 sure is a cord of fibers about a millimeter in diameter connect- 

 ing the caudal portions of the thalami. 



The Basal Ganglia. — A semi-independent group of nerve cells 

 forming a definite mass is known as a ganglion. In the ven- 

 tral portion of the brain lie three pairs of large ganglia, called 

 corpora quadrigemina, thalami, and corpora striata. To study 

 them, the entire dorsal surface of the brain down to and includ- 

 ing the corpus callosum should be removed. 



