NERVOUS SYSTEM. 



109 



Mastoid or sphenoid lobe represents the middle lobe of man; its 

 margins form the transverse fissure and fissure of Sylvius. Its 

 interior forms a cavity, the reflexed portion of the lateral ventricles. 

 The convolutions of the brain are numerous and distinct, but 

 do not call for a detailed description. 



INTERIOR OF THE BRAIN. 



The corpus callosum is the transverse commissure between the 

 hemispheres and seen on separating them. On its top are seen 

 the nerves of Lancisi running on its long axis. Its inferior surface 

 forms the roof of the lateral ventricles separated by the septum 

 lucidum. Its sides end in the interior of its hemispheres. Its 

 hinder border forms the splenium (or pad), turns forward as the 

 genu (or knee), and is continuous with the fornix. Its anterior 

 portion ends in the cerebral substance. 



THE VENTRICLES. 



These are cavities in the interior of the brain as follows 



Lateral Ventricles (two, below the callosum). 



Third Ventricle, between the thalami optici. 



Fourth Ventricle, between the diverging restiform bodies of 

 medulla. (The cavity between the lamellae of the septum lucidum, 

 the fifth ventricle of man, is absent in the horse.) 



LATERAL VENTRICLES. 



Are lined by a serous membrane, the "ependyma," and 

 bounded 



Above, by corpus callosum. 



Below, by corpus striatum, fornix, hippocampus, and choroid 

 plexus. 



Internally, by septum lucidum. 



Externally, by brain structure. 



Anteriorly, continued into cavity of olfactory lobe. 



Posteriorly, continued downward and upward into the mastoid 

 lobe. 



It communicates below and in front by two openings, the 

 foramina of Monro, with the third ventricle. 



The septum lucidum is a thin partition of white matter between 

 the lateral ventricles. 



The fornix, a single body between the third and lateral ventri- 

 cles, is continuous behind with the callosum. It is composed of 

 white matter, terminating behind in two prolongations, the poste- 



