160 • CRANIAL NERVES. 



Lateral Ventricles^ exist in each hemisphere, and contain the corpus 

 striatum and thalamus opticus. Each communicates with the third 

 and its fellow by the foramen of Monro, being separated partially 

 by the septum lucidmn; the roof is formed by the corpus cal- 

 losum. It contains three angular depressions called corivua; the an- 

 terior contains nothing ; the posterior, the hippocampus minor ; and 

 the inferior, the hippocampus major. 



Tlie third ventricle is the space between the thalanii optici. Its 

 roof is formed by the velum interpositum and fornix, and itg floor 

 by the locus perforatus and the tuber cinereum. It is traversed in 

 front by the anterior commissure, a medullary cord extending be- 

 tween the corpora striata, and by the posterior commissure, which ex- 

 tends transversely between the thalami optici. It communicates with 

 the lateral ventricles by the foramen of Monro, and with the fourth 

 by the aqueduct of Sylvius. 



The fourth ventricle is situated between the pons Varolii, cerebel- 

 lum, and medulla oblongata. Its floor is the calamus scriptorius, 

 and its roof is the valve of the brain. Laterally, it is limited by the 

 pia mater and the arachnoid. It communicates with the third. 



The fifth ventricle is situated between the laminae of the septum 

 lucidum. It does not communicate with the other ventricles. 



CEREBELLUM. 



This constitutes about one-sixth of the brain, and is contained be- 

 tween Ihe occiput and tentorium. It is oblong and fla*ttened : its 

 greatest diameter being transverse. It is composed of white and gray 

 substances, the latter of which is external, and apparently arranged in 

 laminae. It is divided by a longitudinal fissure into two lobes ; the su- 

 perior part of which fissure contains a ridge called vermis superior, 

 in advance of which is an elevation called monticulus. In the infe- 

 rior part of this fissure is a smaller ridge called the vermis inferior. 



At the root of the crura cerebelli are two small protuberances ; 

 the one below it, in the erect position, is the lobulus amygdaloides, 

 and the other the lohidus nervi pneumogastrici. 



The valve of the brain is a thin gray plate, extending from the in- 

 ferior surface ef the cerebellum to the corpora restiformia. 



Arbor vitce, a section of either lobe of the cerebellum, presents 

 an arborescent arrangement of the medullary matter. 



Corjous dentatum, is a name applied to a gray mass with serrated 

 edges, in the trunk of this medullary tree. 



CRANIAL NERVES. 



These are nine in number, and are so called from their emerging 

 thrc^ugh the foramina, at the base of the cranium. They are desig- 

 nated by their function, as well as numerically. Their origin is as 

 follows : 



