660 THE BKAIN. 



another is very gradual, not sharp and distinct, and is perhaps especially 

 gradual in passing from the motor region backward to the occipital region. 

 It is not possible to recognize histologically the limit, for instance, of the 

 motor region as determined experimentally. 



In special regions of the brain, for instance in the olfactory bulb, of which 

 we shall speak later on, very great modifications of the general plan may be 

 observed in the cortex. We cannot enter upon these, but may just refer to 

 the cornu ammonis or hippocampus. At the ventral end of the temporal 

 lobe the gyrus hippocampi, the structure of whose cortex follows the general 

 plan, is thrust inward so as to project into the cavity of the descending horn 

 of the lateral ventricle, forming the ridge-like prominence known by the 

 above name. The substance of the cornu ammonis is therefore cortical sub- 

 stance covered on the side of the ventricle by a thin prolongation of the 

 central white matter, which is in turn covered by the ependyma lining the 

 ventricle. A vertical section of this substance shows that while the fifth 

 and fourth layers are reduced to small dimensions, the third layer, that of 

 large pyramidal cells, is well developed, though narrow. The cells are large 

 and remarkably long, and the tapering processes are arranged so regularly 

 as to give rise, especially in stained preparations, to a marked radiate ap- 

 pearance. At the level of the second layer there occurs a large develop- 

 ment of capillary bloodvessels and a scarceness of cells, giving rise to a 

 " lacunar " appearance ; and the first or molecular layer is of some consid- 

 erable thickness. From the prominence of the pyramidal cells in this 

 region, the third layer in the general plan of the cortex has sometimes been 

 spoken of as the " formation of the coruu ammonis." 



565. In the present state of knowledge it is impossible to come to any 

 satisfactory conclusion concerning the meaning of the variety and arrange- 

 ment of the cells and other constituents of the cortex. The cells with their 

 branches, the nerve-fibres and the nerve-fibrils form a network of gray 

 matter which we may compare with the gray matter of the spinal cord 

 ( 492), but which is obviously, as we might expect, far more complex than 

 that is. We may conclude, and experimental observation confirms the con- 

 clusion, that the large pyramidal cells with recognizable axis-cylinder pro- 

 cesses serve as trophic centres for the fibres which appear to start from them. 

 And we may, though with less confidence, explain the large size of these 

 cells in the motor region, by the fact that they give rise to fibres of the 

 pyramidal tract stretching a long way from their origin in the cell, and 

 therefore demanding great nutritive activity on the part of the cell. We 

 may perhaps also conclude that these fibres are efferent, motor fibres, des- 

 tined to carry impulses from the cortex to the peripheral, or at least distant 

 parts. And we may further, with distinctly less confidence, however, assume 

 that the size of the cell is correlated to the energy which has to be expended 

 in the discharge of efferent, motor impulses. If we accept these conclusions 

 we must also bear in mind that such cells, with axis-cylinder processes con- 

 tinued on as fibres, are not limited to, though most abundant in the motor 

 region, but are found in all regions of the cortex ; and we must hence con- 

 clude that impulses, which we must call efferent, proceed from all parts of 

 the cortex. 



It is obvious, however, that the connection of the cortical network of 

 gray matter with the fibres of the white matter is effected in part only, and 

 that a small part, by the method of axis-cylinder processes definitely pro- 

 longed from the cell substance of cells. A part, and probably a greater part, 

 of the fibres sweeping up from the subjacent white matter, whether they be 

 fibres of the pedal and tegrnental systems or callosal or " association " fibres, 

 end in the gray matter in some other way than by bodily being continued 



