234 ANATOMY OF THE CENTRAL NEETOUS SYSTEM. 



deviates from the known phenomena. It may well be imagined that man 

 creates new pathways for himself by cerebral activity, in the sense that the 

 new formation, or the strengthening of pathways already present, may, as 

 the anatomical basis, correspond to the increased ability for execution which 

 exercising of the brain produces. 



As has been previously mentioned, the cerebral cortex is not similarly 

 constructed at all points of the surface. The cortex in the neigJiborhood of the 

 fissura calcarina, for example, is characterized by a preponderance of the 

 small polygonal cells (most of which are clearer) and by a relative poverty 

 of the large pyramidal cells, in addition to the line of Gennari. 



The conformation of the cornu Ammonis deserves special consideration. 

 Near the median line at the base of the brain, the cortex first turns out- 

 ward, then directly inward again, and then curves outward again for a 

 short distance (see Pig. 154). The pyramidal cells of the gyrus hippo- 

 campi, however, do not, as a consequence, pass over immediately into 

 those of the gyrus dentatus. They end, rather, by being irregularly grouped 

 together (at a in Fig. 154), and this irregular mass is then surrounded by 

 the semicircle of regularly arranged cells of the gyrus dentatus. We are 

 now able, withoiit difficulty, to trace the layers of the cornu Ammonis back 

 to the typical layers of the cortex (Meynert and especially Schaffer). ISTever- 

 theless, they present, in their entirety, so many peculiarities that names 

 which they received earlier are still used in describing them. 



In the accompanying illustration let us first follow the cortex from 

 below upward. 



That part of the hippocampal lobe upon which the part that is rolled 

 up rests is designated as the subiculum cornus Ammonis. 



It is covered by an uncommonly well developed layer of tangential 

 fibers, the reticulated arrangement of which is apparent even in the fresh 

 brain. Many of these fibers, traversing the entire thickness of the cortex, 

 appear to extend into the medullary layer of the convolution. At the place 

 where the inrolling begins the layer of tangential fibers becomes thinner, 

 but it accompanies the entire gyrus hippocampi farther and lies, as a glance 

 at the figure must show, directly upon the cortex of the gyrus dentatus. 

 This also possesses a layer of tangential fibers. In man it is difficult 

 to separate the tangential fibers of the gyrus hippocampi from those of 

 the gyrus dentatus; they form together a single layer. The dendrites 

 of the cortical cells extend into this layer exactly as is shown in Fig. 

 152 for the rest of the cortex : on one side, the dendrites from the 

 cortex of the dentatus; on the other, the dendrites from the cortex of the 

 cornu Ammonis. A second large layer of meduUated fibers lies beneath the 

 layer of tangential fibers in the region of the gyrus hippocampi. This 

 curved plate of fibers, the lamina medullaris circumvoluta, is a system of 



