LOBES AND GYRI OF MEDIAL AND TENTORIAL SURFACE. 93 



sulcus, which represents a very important fissure (/. rhinalis] 

 in osmatic animals. The collateral fissure is occasionally inter- 

 rupted by two annectant gyri and divided into a temporal, an 

 occipital and an intermediate part. This signifies a persistence 

 of its embryonic condition. The intermediate portion, some- 

 times assisted by the anterior part, indents the ventricular wall 

 and produces the eminentia collateralis in the inferior horn of 

 the lateral ventricle. 



The inferior temporal sulcus is usually a series of indenta- 

 tions rather than a continuous sulcus (Figs. 26 and 28). It is 

 about equal in extent to the collateral fissure from which it is 

 separated by the fusiform gyrus. It is parallel with the infero- 

 lateral border of the cerebral hemisphere. As the inferior tem- 

 poral gyrus, which forms this border reaches over onto the ten- 

 torial area a variable distance, even in the two sides of the same 

 brain, the position of the inferior temporal sulcus it not constant ; 

 but it is usually one-quarter or one-half inch medial to the border. 



LOBES AND GYRI OF MEDIAL AND TENTORIAL 

 SURFACE. 



The gyri form two concentric rings, interrupted antero-inferiorly 

 at the fossa cerebri lateralis, which encircle the corpus callo- 

 sum and thalamus (Fig. 28). The two rings are separated from 

 one another by a broken fissure, the limbic fissure, made up of 

 the sulcus cinguli (except its marginal end), the subparietal sulcus, 

 and the anterior part of the calcarine and of the collateral fissures. 



Gyrus Fornicatus. The gyrus cinguli and the gyrus hippo- 

 campi, joined together at the posterior border of the corpus cal- 

 losum by the isthmus, and together constituting the gyrus forni- 

 catus, form the central ring. The gyrus cinguli begins anteriorly 

 under the corpus callosum in continuity with the area parolfac- 

 toria anterior to the fossa cerebri lateralis; and the hippocampal 

 terminates as uncus just behind that fossa. The gyrus fornicatus 

 forms the chief part of the limbic lobe. 



The gyrus cinguli is the arched gyrus which is inclosed between 

 the callosal sulcus and the sulcus cinguli, except above the posterior 



