102 Anatomy of the Nervous System 



The hippocampus is composed of three layers of cells, the 

 molecular, pyramidal, and polymorphic, of which the pyramidal 

 layer stands out clearly on account of the rather crowded 

 arrangement of its perikarya. The dentate gyrus is likewise 

 composed of three layers, in this case molecular, granular, and 

 polymorphic. It runs along the edge of the hippocampus and 

 is folded around it, the two portions overlapping considerably. 

 The cells of the granular layer are very closely crowded, 

 making it a conspicuous object in sections where they are 

 stained. The functions of the fascia dentata are chiefly 

 reception and local coordination. Most of the ax^ons of the 

 granule cells pass to the hippocampus proper, which gives rise 

 chiefly to commissural and descending projection fibres. 



The afferent fibres to the hippocampal formation are 

 divisible into medial and lateral groups. The former arise 

 from the various parts of the secondary olfactory area and 

 the septum and make up a considerable part of the fibre 

 system which has already been noted in the latter structure. 

 They are distributed to the hippocampal cortex in various 

 ways, but chiefly through the fimbria and alveus. The 

 lateral group, or external hippocampal fibres (fibrae hippo- 

 campales externae)^ arise from both praepyriform and pyriform 

 areas and either join the alveus or run into the surface of the 

 molecular layer of the hippocampus. Non-olfactory fibres 

 enter the hippocampus directly from neighbouring parts of 

 the neopallium. 



Efferent fibres from the hippocampus run anteriorly 

 through the fimbria into the fornix. The latter forms a 

 sweeping curve anteriorly {body of the fornix), which in the 

 rat is almost absent, as the dorsal end of the hippocampus 

 and the fimbria extend forward practically to the lamina 

 terminalis (Fig. 8) in the position otherwise occupied by the 

 body of the fornix (Pis. XIX. -XXI.). The fornix now turns 

 ventrally into the lamina terminalis, where it passes ventrally 

 and then posteriorly {column of the fornix), proceeding back 



