TH/, CORTEX OF THE FORE-BRAIN, ETC. 67 



arise from the involution have, unfortunately, received special 

 names. As we, however, know what portions of the general 

 cortex arc represented by the different structures of the cornu 

 ammonis, it is better to regard the latter genetically and not dis- 

 turb our comprehension of its arrangement by using the old 

 names. Let us follow the cortex from below upward in the ac- 

 companying figure. You see that the superficial layer of medul- 

 lary fibres becomes thickened far ventrad of the point where it is 

 rolled in. Nowhere else in the brain so well as at this point can 

 it be seen that these fibres, which in great part run in the 

 longitudinal axis of the brain, originate in the radiating fibres 

 below. Near the gyrus dentatus a portion of these fibres pass 

 to a somewhat deeper layer of the cortex, while the rest remain 

 superficial and become blended with the adjoining analogous 

 layer of that structure. Here, as everywhere else, the radiating 

 fibres pass inward from the cortex. In the vicinity of the ^rFs Q^V^- 

 i, however, so few of these fibres arise that they 



not form a thick medullary deposit underneath the cortex, but 

 only a thin layer. This thin coating covers the side of the cornu 

 ammonis which is toward the ventricle, and at the tip of the 

 fold is gathered up into a medullary bundle, no longer covered 

 with gray matter. This is the fornix. Into it pass also the few 

 fibres which spring from the little gyrus dentatus. 



The course of many of the cortical fibres differs in the 

 cornu ammonis from their course in other regions of the cortex. 

 Inasmuch, however, as the origin and significance of the whole 



7 O C> 



net-work shown in Fig. 37 is so uncertain, you will not be 

 specially interested in the analogous structures of the cornu 

 ammonis. Of the greatest importance was the discovery of 

 Tuczeck, that in progressive paralysis of the insane the net-work 

 of fibres in layer 1 is destroyed, and that the fibres in the deeper 

 layers, down to layer ^, successively disappear. 



The nerve-fibres of the cortex receive their medullary in- 

 vestment at a very late period. This takes place first in the 

 superior parietal lobe during the ninth fcetal month. In the 



