THE NERVOUS SYSTEM. 301 



the calamus seriptorius and the fasciculi teretes; antcriarli/, the pons 

 Yarolii and medulla oblongnta : imxtcriorli/, the cerebellum; later nil u, 

 the processus a cerebello ad testes, the restiform bodies, and posterior 

 pyramids of the medulla. 



Its lining membrane is continuous with that of the third 

 ventricle. The gray matter of the floor of the fourth ventricle 

 presents masses for the origin of certain cranial nerves. 



THE FIFTH VENTRICLE is a narrow fissure, formed within 

 the two laminae of the septum lucidum, heing originally a part 

 of the great longitudinal fissure. It is bounded : 



Above, by the under surface of the corpus callosum; 

 Below, by the anterior part of the fornix; 



Laterally, by the lateral ventricles, from which it is separated by 

 the septum lucidum. 



It is not lined with epithelium. 



THE MESEXCEPHALON includes those portions of the brain 

 substance which connect the cerebrum, cerebellum, and medulla 

 oblongata together, and comprises the following structures : 



Crura cerebri, before described (ante, p. 294). 



The valve of Vieussens, or anterior medullary velum, is a 

 thin layer of white matter stretched between the processes e 

 cerebello ad testes, and forming the roof of the iter a tertio ad 

 quartum ventriculum. It presents an elevated ridge descending 

 on its upper part from the corpora quadrigemina the frcenulum. 



Corpora or tubercula quadrigemina, or optic lobes, are four 

 spherical eminences, placed in pairs above the valve of Vieussens, 

 and behind the third ventricle, beneath the posterior border of 

 the corpus callosum. 



The two anterior are called the nates, the two posterior the 

 testes. The brachia (anterior and posterior) are two white 

 cords connecting them with the optic thalamus and beginning 

 of the optic tracts. 



The processus a cerebello ad testes connect them with the 

 cerebellum. 



To the outer side of the optic lobes are two small masses 

 called the corpus geniculatum externum and internum. 



The pineal gland, or epiphysis cerebri, is a small, reddish, 

 conical body resting upon and between the nates. It represents 

 the rudiment of a median eye of certain extinct lizards and am- 

 phibia (Spencer), well marked in Hatteria. It is attached to 

 the cerebrum by its two peduncles, and is held in position by a 

 fold of the pia mater. It contains a cavity, filled with a viscid 

 fluid and secretory matter composed of phosphates of lime, mag- 

 nesia, and ammonia the acorvulus cerebri. 



