54 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



As I showed you before, the fornix forms the margin of 

 the hemispheres. The first convolution outside of this margin 

 a convolution, therefore, which lies close to the fornix is 

 the gyrus hippocampi just mentioned. To the other side of 

 it lies the cavity of the ventricle, the inferior horn. At this 

 point the ventricle is separated from the general cavity of 

 the skull by a thin membrane, bearing the continuation of the 

 choroid plexus, which is attached to the fornix throughout its 

 entire course. 



The gyrus hippocampi may be regarded as the marginal 

 convolution of the hemisphere. This margin, a long, thin, white 

 stripe, is directly continuous with the fornix above. It is called 

 the fimbria (Fig. 19, F.I}.. 



The gyrus hippocampi is pushed out into the cavity of the 

 inferior horn by a fissure on its outer surface, the fissura hip- 

 pocampi. The swelling which is thus produced and which ex- 

 tends along the whole floor of the inferior horn has for ages 

 been called the cornu ammonis, or pes hippocampi major. 



From the fact that the cortex of the gyrus hippocampi is 

 doubled in by the fissura hippocampi before it ceases and leaves 

 the medullary matter bare, a peculiar and complicated appear- 

 ance is presented on cross-section. Over the general surface of 

 the brain the gray matter is continuous, as shown in Fig. 33 A ; 

 but on the gyrus hippocampi it ceases close to the ventricle, as 

 shown in Fig. 33 B, and leaves the white, somewhat curved 

 edge (the fimbria) free. The bending or doubling in which 

 it undergoes before it ceases is shown in Fig. 33 B. Between 

 the gyrus hippocampi and the free medullary margin of the 

 hemisphere (fimbria fornix) there lies a small convolution (pur- 

 posely not mentioned before), which passes from the end of the 

 corpus callosum to the apex of the temporal lobe, and therefore 

 takes part in the configuration of the cornu ammonis. You 

 will please look up this convolution in the sagittal section (Fig. 

 32) in order to make clear its situation in relation to the fornix 

 and the cornu ammonis. It is called the gyrus dentatus, or 



