200 ANATOMY OF THE CENTRAL NERTOUS SYSTEM. 



fimbria. Fig. 134c is intended to show this and the infolding which the cortex un- 

 dei'goes before it ends. Between the gyrus hippocampi and the free, medullary edge 

 of the hemisphere (fimbria-^fornix), there still lies, however, a small -convolution, 

 which has purposely not been mentioned heretofore. It passes downward 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 also. Locate this narrow gyrus, designated 

 as the gyrus dcntutus or -fascia dentata, on the sagittal section previously demon- 

 strated, in order to make its relation to the fornix and the gyrus hippocampi per- 

 fectly clear. It lies, as is there seen, just in front of the inrolling of the gyrus 

 hippocampi, produced by the similarly named fissure. A cross-section of the gyrus 

 dentatus is, therefore, not represented by Fig. 1346, but more correctly by Fig. 134c. 



The cornu Ammonis is, therefore, the bulging that arises in the ventricle through 

 the infolding of the gyrus hippocampi by the fissura hippocampi. The complicated 

 appearance of the cornu Ammonis, when seen in transverse section, is due to the 

 fact that the cortex of the gyrus ceases just at this place, and that the fimbria and 

 gyrus dentatus run along over this infolding. 



The relation of the gyrus hippocampi to the inferior horn of the lateral ven- 

 tricle becomes clearest, if their cross-sections are traced in the large sections of the 

 brain reproduced in Figs. 175, 185, 186, and 187. 



The gyrus fornicatus and its continuation, the gyrus hippocampi, are formed 

 rather early in the embryo. Dorsal to the margin of the hemisphere (arch of the 

 fornix) there appears in all mammals a fissure which, running parallel with the 

 fornix, passes down \\ith it into the temporal lobe. It is called the marginal fissure, 

 or fissura liniblca, and the gyrus left between it and the fornix, the marginal con- 

 volution. The fibers of the corpus callosum pass between this gyrus and the fornix 

 in the more anterior region of the brain, and the convolution is there known as the 

 gyrus fornicatus. The more posterior portion of the marginal convolution, however, 

 — the portion designated as the gyrus hippocampi, — borders almost directly on the 

 fornix. In most mammals the corpus callosum and the gyrus fornicatus are very 

 short. 



If the upper surface of the corpus callosum is again examined, a thin, 

 gray, longitudinal line will be seen upon it on each side (Fig. 124, Lt). That 

 line — the stria longitudi nails Lancisi — is the continuation of the gyrus den- 

 tatus: a convolution atrophied even in the cornu Ammonis. 



At the posterior end of the corpus callosum a short convolution is some- 

 times seen passing in a direction toward the fornix, with which it unites. 

 It is the gyrus callosus, which occurs in man as a very atrophic structure 

 only, and is not at all constant. 



Find the gyrus uncinatus on the fresh brain internal to the apex of the 

 temporal lobe, and from there follow the gyrus hippocampi upward. Then 

 find the arch of the fornix, which is easily done, above the posterior portion 

 of the thalamus, and note that it passes over into the fimbria, which is 

 visible as a white, medullary line almost to the apex of the cornu Ammonis. 

 Finally, make a frontal section, which will show the relation of the structures 

 named to the inferior horn. 



On the base of the brain only a few important fissures are found in addi- 



