1 86 ELEMENTS OF MAMMALIAN ANATOMY. 



median cornu of the lateral ventricle (Fig. 96). The free 

 lateral margin of the posterior pillar or crus of the fornix is 

 the ftmbria, or tcenia hippocampi. Adjacent to the median 

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

 lucidum, 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. Between each anterior pillar of 

 the fornix and the optic thalamus is a cleft, the -foramen 

 of Monro, leading from the third ventricle, laterad of the 

 septum lucidum, into the lateral ventricle. 



The three remaining commissures are known according 

 to their location as the anterior, middle, and posterior. 

 The anterior commissure (Figs. 95 and 98) perforates the 

 corpora striata, extending across the median line imme- 

 diately craniad of the anterior pillars of the fornix. It is 

 about two millimeters in diameter. The middle commis- 

 sure, or massa intermedia, lies between the optic thalami. 

 It is sometimes called the soft or gray commissure. It is 

 nearly one centimeter in diameter and passes through the 

 third ventricle (Fig. 95). The posterior commissure is a 

 cord of fibers about a millimeter in diameter connecting the 

 caudal portions of the optic thalami. 



The Basal Ganglia. A semi-independent group of 

 nerve cells forming a definite mass is known as a ganglion. 

 In the ventral portion of the brain lie three pairs of large 

 ganglia, called corpora quadrigemina, optic thalami, and 

 corpora striata. To study them, the entire dorsal surface 

 of the brain down to and including the corpus callosum 

 should be removed. 



The corpora quadrigemina (Fig. 96) forming the dorsal 

 part of the mesencephalon lie craniad of the medulla, and 

 consist of an anterior pair and a posterior pair. The for- 

 mer are known as the superior colliculi and the latter as 



