52 DK. G. ELLIOT SMITH ON THE 



A coronal section tlirongh the cerebrum, just in front of that represented in the first 

 figure, presents a marked change in the appearance of the hippocampal region, Avhich 

 is represented upon an enlarged scale in fig. 2. The hijipocarapus itself, as well 

 as the fascia dentata, have undergone practically no change, but a huge bundle of 

 medullated fibres from the upper part of the fimbria is now exposed in the act of crossing 

 the middle line. This is the posterior exti'emity of what — to avoid confusion of terms — 

 will for the present be termed the commissura dorsalis (c.i).). 



It would be foi-eign to the scope of this work to descril)e in detail the histology of the 

 hippocampal formation, seeing that the minute structure of the Metatherian hippocampus 

 is identical with that of the Eutherian, which Sala, Schaff'er, Ramon y Cajal, and 

 von KoUiker*, among many others, have so carefully and minutely described Avithin 

 recent years. But I wish to call attention to certain features which are intimately 

 associated with the evolution of the structure we are primarily considering. 



Lying upon the upper surface of the " dorsal commissure " (c.d.) there is a complicated 

 epithelial fold the outline of which is roughly triangular, and which projects upward into 

 the great cleft between the two cerebral hemispheres. The fascia dentata {f.d.) lies in 

 contact with this epithelial (choroidal) fold on either side, and may be easily recognized 

 by certain distinctive features. Thus it possesses a very thickly -packed column of small 

 cells — the stratum gramdosum (s.fjr.) — the nuclei of which stain very deeply, lying parallel 

 to the surface, from which they are separated by a clear layer almost devoid of cells — the 

 stratum molecuJare. The fascia dentata — consisting of stratum granulosum and stratum 

 moleculare — is folded in a peculiarly characteristic manner. Its mesial extremity is 

 completely bent upon itself, so that the stratum moleculare or superficial layer rests 

 upon the dorsal aspect of the dorsal commissure. The lateral extremity of the fascia 

 dentata undergoes a corresponding bending, the concavity of which is directed toward the 

 mesial plane. It would appear that the fascia dentata had continued to increase in 

 breadth whilst its mesial and lateral extremities were relatively fixed, and, to accommodate 

 itself to tliese conditions, it bad bulged out into a pouch, as it were, and at the same time 

 become flattened by the resistance of the general cortex lying immediately above it. Such 

 factors, in reality, seem to have been at work, as I have recently pointed out t. The 

 fissure which separates the fascia dentata from the general cortex where their resj^ective 

 superficial layers come into contact is ihefissura hipiMcampi (f.h.). 



The concavity of this pouch of fascia dentata is occupied l)y scattered nerve-cells, many 

 of which give origin to the brush of medullated fibres which are to be seen in the figure, 

 collecting near the inner margin of the fascia dentata, to lose themselves in the huge 

 mass of medullated fibres lying immediately below. 



The hippocampus proper projects as a large rounded swelling into the lateral ventricle. 

 This swelling is covered by a thick layer of medullated fibres — the so-called ulceus (alv.). 

 These fibres almost all pursue an obliqiie course, so that in a coronal section only short 

 pieces of ^fibre are seen massed together. In the first figure most of the fibres of the 



* ' Gewebelehre des "Munsohen,' Bd. ii. zwciter Hiilt'te, IS'JG. 



t '■ The Fascia Dentata,"' Auatomischer Aiizeiger, xii. Bd. Nr. 4 iind .5, 1895, p. 119. 



