666 THE NERVOUS SYSTEM. 



responds with the upper part of the anterior commissure. The outer surface of 

 each lamina is directed toward the lateral ventricle, and is covered by the ependyrua 

 of that cavity, while its mesial surface bounds the cavity of the fifth ventricle. 



Fifth Ventricle. The fifth ventricle was originally a part of the great longi- 

 tudinal fissure, which has become shut off by the union of the hemispheres in the 

 formation of the corpus callosum above and the fornix below. Each half of the 

 septum is therefore formed by the median wall of the hemisphere, and consists of 

 an internal layer of gray matter, derived from the gray matter of the cortex, and 

 an external layer of white substance continuous with the white matter of the cere- 

 bral hemispheres. The fifth ventricle differs from the other ventricles of the brain, 

 inasmuch as it is not developed from the cavity of the cerebral vesicles, it is not 

 lined by ciliated epithelium but by altered pia mater, and it does not communicate 

 with the general ventricular cavity ; further, the fluid it contains is of the nature 

 of lymph. 



The structures on the floor of the descending horn of the lateral ventricle will 

 now be described. 



The hippocampus major, or cornu Ammonis (Fig. 356), is a white eminence, 

 about two inches in length, of a curved elongated form, extending throughout the 



major. j 



Eminentia 

 collateralis. 



Hippocampus \ EflH W i dentate. 



f Uncinate 



Gray matter ) 1 convolution. 



of the [ -V ^ -( Uncinate 



hippocampus. j iJSr \ process. 



FIG. 356. Transverse section of the middle horn of the lateral ventricle. (From a drawing by Mr. F. A. 

 Barton.) 



entire length of the floor of the descending horn of the lateral ventricle. At its 

 lower extremity it becomes enlarged, and presenting two or three rounded 

 elevations with intervening depressions, it resembles the paw of an animal, 

 and is called the pes hippocampi. If a transverse section is made through the 

 hippocampus major, it will be seen that this eminence is produced by the folding 

 of the cortex of the brain to form the dentate (hippocampal) sulcus. To the outer 

 side and parallel with the hippocampus major an elongated eminence, the eminentia 

 collateralis, is frequently recognized. It corresponds with the middle part of the 

 collateral fissure, and its size depends on the direction and depth of this fissure. 

 The main mass of the hippocampus major consists of gray matter, but on its ven- 

 tricular surface is a thin layer of white matter, known as the alveus, which is con- 

 tinuous with the corpus fimbriatum of the fornix and is covered by the ependyma 

 of the ventricle. Dr. J. G. Macarthy, of McGill University, Montreal, has 

 shown * that, if the alveus and superficial strata of gray matter be reflected from 

 the surface of the hippocampus by an incision carried along its convexity, the 

 "core " of the hippocampus, as he terms it, presents in many cases a corrugated or 

 crimped appearance. 



1 Journal of Anat. and Pkys., vol. xxiii., 1899. 



