296 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



ceeds forward to the extremity of the hemisphere, where it meets the apex of the 

 occipito-temporal lobe from which it is separated by the fissure of Sylvius, which also 

 separates these two lobes for a considerable distance on the lateral surface. They 

 run into each other or become continuous back of the position of the posterior ex- 

 tremity of the horizontal portion of the fissure of Sylvius around which they arch. 

 On the mesial surface this lobe is separated from the mesial surface of the occi- 

 pito-temporal posteriorly, by the anterior portion of the calcarine fissure, and pro- 

 ceeds anteriorly surrounding the corpus callosum. 



2. The occipito-temporal lobe has corresponding relations. Posteriorly it is 

 separated from the occipital lobe by the inferior branch of the primary occipital 

 arch, 02, fissura occipitalis inferior. It passes forward following the fornix and is 

 separated from the occipitofrontal as described above. 



3. The occipital lobe forms the posterior part of the undivided hemisphere and 

 is a single symmetrical lobe cut off from the preceding by the development of two 

 arching fissures, 01 and 02, as previously mentioned. 



4. The lobulus centralis or Island of Reil is situated at the junction of the 

 occipito-frontal and temporal lobes anteriorly, is similarly related to each, and is 

 covered in the adult condition more or less completely by the so-called operculum, 

 and below by the superior temporal convolution. 



Owing to its simpler character and the retention of its primitive relations to 

 the remaining portions of the hemisphere, we shall first proceed to discuss the 

 fissures and convolutions of the occipito-temporal lobe. 



OCCIPITO-TEMPORAL LOBE. 



The occipito-temporal lobe, OT, occupies the temporal fossa of the skull and is 

 situated partly above the cerebellum, resting upon the anterior portion of the 

 tentorium. Its general boundaries and connections have been already described. 



Fissures and Convolutions. 



The convolutions and fissures of the occipito-temporal lobe are comparatively 

 simple and can be easily distinguished. On the lateral surface three convolutions, 

 OT 1 or S. O.T; OT 2 or M. 0. F., and OT 3 or I. 0. F (see plates) separated from each 

 other by two fissures (o.t. 1 ) and (o. t 2 ) are found. The fissure o. t 1 is the superior 

 temporal, sulcus temporalis superior, the scissure paralleie of Gratiolet, antero- 

 temporal sulcus of Huxley. This is the most persistent and constant fissure of the 

 lateral surface. It is well marked in all the Simiadae except the marmosets. In 

 Chrysothrix, Plate XXXVIII, figs. 6 and 9, it is well defined. In the negro I 

 have found it straighter and less tortuous than in the Caucasian. In a chimpanzee 

 it was found by Turner 1 to be continuous with the Sylvian above, so that the 

 Sylvian had the appearance at first of mounting much higher than normal, as this 



1 Notes, more especially on the Bridging Convolutions in the Brain of the Chimpanzee. Proc. Roy. 

 Soc. Edinburgh. 



