76 ATLAS OF NERVE CELLS 



THE HIPPOCAMPUS 



The hippocampus is a peculiar structure which has attracted the attention of all anato- 

 mists who have studied the brain. It lies deep upon the base on the inner surface of the 

 tempero-sphenoidal lobe, its free inner surface projecting into the descending horn of the 

 ventricle. The peculiar appearance of the anterior portion known as Ammon's Horn is due 

 to a folding of the convolution upon itself in such a manner that the outer layer of one 

 portion of the cortex, being folded backward, comes in contact with the outer layer of another 

 portion of the cortex, and the free extremity is rolled outward in a sort of spiral curve. This 

 peculiar configuration is well shown in Plate LI. 



It is not necessary to give a very detailed account of this structure, as its physiological 

 significance is not yet known. It is much more fully developed in the lower animals than 

 it is in man. It is easily possible to distinguish a number of layers in the cortex, and these 

 may be enumerated as follows (from right to left in the figure): 



First. A layer of epithelium upon which lies the choroid plexus in the descending horn 

 of the lateral ventricle. This forms the outer boundary of the alveus. 



Second. The second layer is the layer of white matter, really made up of the neuraxons 

 issuing from the base of the pyramidal cells, but deflected by the curving of the convolution 

 into a zone of horizontal fibres. This is known as the Alveus. 



Third. A thin layer containing numerous polygonal cells which are similar to those of 

 the fourth layer of the cortex. This has been called the stratum oriens. 



Fourth. A layer of pyramidal cells of various shapes and sizes having many dendrites and 

 long apical processes which divide and branch as they enter the fourth layer. This corresponds 

 to the third and second layers of other cortical regions fused together. 



Fifth. A layer made up of numerous fine fibres passing in all directions, and of many 

 cells. This is really the superficial layer of the cortex, and is fused with a superficial layer 

 of the other part of the cortex, which is folded upon this layer; so that this broad layer 

 represents the union of two superficial cortical layers. It has been named the stratum lacu- 

 nosum. It contains many Cajal cells and cells of Golgi's second type. 



Sixth. A layer of pyramidal cells with their bases directed away from the layer of fine 

 fibres ; thus lying in exactly an opposite direction to the cells of the fourth layer. 



Seventh. A layer of white matter made up of the neuraxons coming out of the base of 

 these pyramidal cells. The fibres issuing from the seventh layer gather into a mass and 



