1032 HISTOLOGY OF CORTEX. [BOOK in. 



known by the above name. The substance of the cornu ammonis 

 is therefore cortical substance 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 shews 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 pre- 

 parations to a marked radiate appearance. At the level of the 

 second layer there occurs a large development of capillary blood 

 vessels and a scarceness of cells, giving rise to a ' lacunar ' 

 appearance ; and the first or molecular layer is of some con- 

 siderable 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 cornu 

 ammonis." 



652. In the present state of knowledge it is impossible to 

 come to any satisfactory conclusion concerning the meaning of the 

 variety and arrangement 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 grey matter which we may compare 

 with the grey matter of the spinal cord ( 579) but which is 

 obviously, as we might expect, far more complex than that is. 

 We may conclude, and experimental observation confirms the 

 conclusion, that the large pyramidal cells with recognisable axis 

 cylinder processes 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, destined to carry impulses from the cortex to peri- 

 pheral or at least distant parts. And we may further, with 

 however distinctly less confidence, 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 continued 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 conclude that im- 

 pulses, which we must call efferent, proceed from all parts of the 

 cortex. 



It is obvious however that the connection of the cortical net- 

 work of grey 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 prolonged from the cell substance of cells. A 



