516 THE NERVOUS SYSTEM 



1. The molecular fibre layer. 



2. The grouped polymorphous cell layer. 



3. The layer of irregular pyramidal cells. 



4. The inner polymorphous cell layer. 



Ammon's horn presents a peculiar formation, which may he 

 considered as forming a transition area between the cerebral cortex 

 and the very thin fascia dentata. The cornu Ammonis is charac- 

 terized by the special prominence of but one cell type, the py- 

 ramidal, this cell layer forming the greater part of its thickness. 



Its basal layer, the alveus homologue of the cerebral medulla 

 is continuous with the white matter of the gyrus hippocampus. 

 Within the alveus is a thin molecular layer, the stratum oviens, 

 homologous with the polymorphous cell layer ; it contains a very 

 few small fusiform cells. The basal processes of the pyramidal 

 cells penetrate this layer to spread out within the alveus. 



Next to the stratum oviens is the broad pyramidal cell layer, 

 its cells lying in the basal portion of the layer and sending their 

 processes toward the alveus. From the apices of these cells a 

 second set of processes pass toward the outer molecular layer, their 

 thicker stem portions producing the radial appearance from which 

 this part has been called the stratum radiatum. 



The small pyramidal and outer polymorphous cell layers are 

 merely represented by an unusually vascular, molecular layer con- 

 taining a very few small nerve cells. The abundance of small 

 anastomosing blood vessels in this layer gives it a lacunar appear- 

 ance, hence its name the stratum lacunosum. 



The true molecular layer is the next beyond the lacunar stratum, 

 and is similar in structure to its homologue in other parts of the 

 cerebral cortex. On the surface of this layer, replacing the pia 

 mater, is the lamina medullaris involuta, which contains the ter- 

 minal dendrites of the pyramidal cells ; it is therefore homologous 

 with the superficial tangential fibres of the molecular layer in other 

 portions of the cerebral cortex. 



To recapitulate, the layers of Ammon's horn are as follows : 



1. Lamina medullaris involuta. 5. Pyramidal cell layer. 



2. Molecular layer. 6. Stratum oviens. 



3. Stratum lacunosum. 7. Alveus. 



4. Stratum radiatum. 



THE CEREBELLUM. The cortex of the cerebellum, like other 

 portions of grey matter, consists of nerve cells with their naked 

 processes, together with their supporting neuroglia. The grey 



