288 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



it as of fundamental morphological significance but regard its appearance as the 

 result of the excessive growth of the frontal region in Man and the few excep- 

 tional cases of Anthropomorphs in which it is found. 



The appearance and extent of development of the Sylvian fissure in the 

 Arctopithecini, Lemuridae and the Simiadse may be seen in Plates XXXVII, 

 XXXVIII, XXXIX. 



MESIAL AECHED FISSUEE. 



(Fissura arcuatus, medialis or interna.) 



The mesial arched fissure (fissura callosalis and fissura hippocampi) is, with 

 the Sylvian, the most constant of the fissures of the hemisphere and I consider it 

 of the greatest importance as regards an understanding of the true character 

 and arrangement of the other fissures and convolutions. Previous writers do 

 not appear to have considered this fissure, as a whole, as primary and fundamen- 

 tal. Indeed, curiously enough, many writers do not describe its upper branch, 

 nor do they give it a special name. This upper branch arches around the 

 corpus callosum and Owen has called it, therefore, the fissura corporis callosi or 

 callosal fissure, and Arnold described it as part of his arched fissure (Bogenfurche). 

 The lower branch has, however, been recognized and described by all recent 

 investigators under the name of the fissura hippocampi, anterior part of the 

 scissure des hippocampes (Gratiolet), fissura dentatus of Huxley and the English 

 anatomists. It extends downward and forward, producing the elevation in the 

 descending horn known as the hippocampus major, and at its lower extremity 

 separates the gyrus uncinatus (see figures) from the rest of the cerebral surface. 

 Shortly after the formation of the above fissure there appears in connection with 

 the evolution of the occipital lobe, a posterior horizontally directed fissure which 

 is continuous with the mesial arched, see figs. 4 and 6, Plate XXXV, and is known 

 as the fissura calcarina. It forms one of the branches of what I have termed 

 as a whole the fissura tri-radiatus and follows the course, as we have previously 

 mentioned, of the posterior horn producing in it the prominence known as the 

 calcar avis or hippocampus minor; whence the name calcarine given to it by 

 Huxley. It represents the posterior part of the scissure des hippocampes of 

 Gratiolet, the fissura horizontalis of Pansch, the fissura posterior or horizontalis 

 of R. Wagner, the fissura hippocampi of Bischoff. This is one of the most con- 

 stant fissures of the hemisphere and is found well developed in all Primate 

 brains with the exception of some of the Lemuridae. It appears in the human 

 foetus as a well marked fissure when the rest of the hemisphere is completely 

 smooth. It may be seen, fig. 4, Plate XXXV, in the brain of a foetus of three 

 and a half months, well developed. This fissure in the Simiadse is continuous 

 anteriorly with the fissure, m. a., and Gratiolet called the fissure extending ±rom 

 the end of the occipital lobe to the extremity of the temporal, the scissure des 

 hippocampes, thus distinguishing but one fissure instead of two. He applied the 

 same nomenclature to the brain of Man. In the human brain, however, the poste- 



