

THE FORE-BRAIN OR PROSENCEPHALON 845 



radiata; its fibers radiate in a fan-like manner and pass into the occipital and tem- 

 poral lobes lateral to the posterior and inferior cornua. Dejerine regards the fibers 

 of the tapetum as being derived from this fasciculus, and not from the corpus 

 callosum. 



(0 The fornix connects the hippocampal gyrus with the corpus mammillare 

 and, by means of the thalamomammillary fasciculus, witn the thalamus (see page 

 839). Through the fibers of the hippocampal commissure it probably also unites 

 the opposite hippocampal gyri. 



The gray substance of the hemisphere is divided into: (1) that of the cerebral 

 cortex, and (2) that of the caudate nucleus, the lentiform nucleus, the claustrum, 

 and the nucleus amygdalse. 



Structure of the Cerebral Cortex (Fig. 754). The cerebral cortex differs in thickness and 

 structure in different parts of the hemisphere. It is thinner in the occipital region than in the 

 anterior and posterior central gyri, and it is also much thinner at the bottom of the sulci than 

 an the top of the gyri. Again, the minute structure of the anterior central differs from that of 

 the posterior central gyrus, and areas possessing a specialized type of cortex can be mapped out 

 in the occipital lobe. 



On examining a section of the cortex with a lens, it is seen to consist of alternating white and 

 gray layers thus disposed from the surface inward: (1) a thin layer of white substance; (2) a 

 layer of gray substance; (3) a second white layer (outer band of Baillarger or band of Gennari); 

 (4) a second gray layer; (5) a third white layer (inner band of Baillarger); (6) a third gray layer, 

 which rests on the medullary substance of the gyrus. 



The cortex is made up of nerve cells of varying size and shape, and of nerve fibers which are 

 either medullated or naked axis- cylinders, imbedded in a matrix of neuroglia. 



Nerve Cells. According to Cajal, the nerve cells are arranged in four layers, named from the 

 surface inward as follows: (1) the molecular layer, (2) the layer of small pyramidal cells, (3) 

 the layer of large pyramidal cells, (4) the layer of polymorphous cells. 



The Molecular Layer. In this layer the cells are polygonal, triangular, or fusiform in shape. 

 Each polygonal cell gives off some four or five dendrites, while its axon may arise directly from 

 the cell or from one of its dendrites. Each triangular cell gives off two or three dendrites, from 

 one of which the axon arises. The fusiform cells are placed with their long axes parallel to the 

 surface and are mostly bipolar, each pole being prolonged into a dendrite, which runs horizontally 

 for some distance and furnishes ascending branches. Their axons, two or three in number, arise 

 from the dendrites, and, like them, take a horizontal course, giving off numerous ascending, 

 collaterals. The distribution of the axons and dendrites of all three sets of cells is limited to the 

 molecular layer. 



The Layer of Small and the Layer of Large Pyramidal Cells. The cells in these two layers 

 may be studied together, since, with the exception of the difference in size and the more super- 

 ficial position of the smaller cells, they resemble each other. The average length of the small 

 cells is from 10 to 15MJ that of the large cells from 20 to 30M. The body of each cell is pyramidal 

 in shape, its base being directed to the deeper parts and its apex toward the surface. It contains 

 granular pigment, and stains deeply with ordinary reagents. The nucleus is of large size, and 

 round or oval in shape. The base of the cell gives off the axis cylinder, and this runs into the 

 central white substance, giving off collaterals in its course, and is distributed as a projection, 

 commissural, or association fiber. The apical and basal parts of the cell give off dendrites; the 

 apical dendrite is directed toward the surface, and ends in the molecular layer by dividing into 

 numerous branches, all of which may be seen, when prepared by the silver or methylene-blue 

 method, to be studded with projecting bristle-like processes. The largest pyramidal cells are 

 found in the upper part of the anterior central gyrus and in the paracentral lobule; they are 

 often arranged in groups or nests of from three to five, and are named the giant cells of Betz. 

 In the former situation they may exceed 50/* in length and 40M in breadth, while in the para- 

 central lobule they may attain a length of 65M. 



Layer of Polymorphous Cells. The cells in this layer, as their name implies, are very irregular 

 in contour; they may be fusiform, oval, triangular, or star-shaped. Their dendrites are directed 

 outward, but do not reach so far as the molecular layer; their axons pass into the subjacent white 

 matter. 



There are two other kinds of cells in the cerebral cortex. They are: (a) the cells of Golgi, 

 the axons of which divide immediately after their origins into a large number of branches, which 

 are directed toward the surface of the cortex; (6) the cells of Martinotti, which are chiefly found 

 in the polymorphous layer; their dendrites are short, and may have an ascending or descending 

 course, while their axons pass out into the molecular layer and form an extensive horizontal 

 arborization. 



