198 ELEMENTS OF MAMMALIAN ANATOMY. 



The three remaining commissures are known according 

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

 The anterior commissure (Figs. 92 and 95) perforates the 

 corpora striata, extending across the median line imme- 

 diately cephalad of the anterior pillars of the fornix. It 

 is about two millimeters in diameter. The middle com- 

 missure, or massa intermedia, lies between the optic 

 thalami. It is sometimes called the soft or gray com- 

 missure. It is nearly one centimeter in diameter and 

 passes through the third ventricle (Fig. 92). The pos- 

 terior 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. 93) lie cephalad of the 

 medulla, and consist of an anterior pair, the nates, and 

 a posterior pair, the testes. A narrow canal, the iter, 

 or aqueduct of Sylvius, leads from the fourth ventricle 

 through the corpora quadrigemina (Fig. 92) from the 

 fourth ventricle to the third ventricle. The nates lie 

 nearer to the midline than the testes, which are slightly 

 separated by a depression occupied by the middle portion 

 of the central lobe of the cerebellum. The testes are 

 united by a white commissure. The posterior commis- 

 sure of the brain unites the cephalic portions of the nates 

 (Fig. 92). Its cut end may be seen ventrad to the base 

 of the pineal gland. 



Laterally each pair of the corpora quadrigemina is 



