MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 283 



fissura callosalis or fissure of the anterior horn, and an inferior portion, the fissura 

 hippocampi or fissure of the descending horn. The posterior backward prolonga- 

 tion remains as the fissura calcarina or fissure of the posterior horn. 



] Fiss. callosalis 

 i 



( Fiss. arc » at us nied. ^. . . „ . 



Fiss. radiatus \ J Fibs, hippocampi 



1 Fiss. calcarina 



As development proceeds beyond the fourth month the brain, as we have 

 alread}^ mentioned, again becomes almost entirely smooth, and we see the con- 

 ditions produced represented in Plate XXXV, figs. 5 and 6, of the human foetal 

 brain at the middle of the fifth month. We have here what I believe is not only 

 the fundamental morphological plan of the human brain, but also that of all the 

 Primates, with the possible exception of some of the Lemuridre. It is for these 

 reasons that I separate the Primate brain into the following divisions, based upon 

 the internal structure of the cerebrum itself, as shown by its ventricles and upon 

 the appearance of certain fundamental and primary fissures. 



LOBES. 



1. The occipitofrontal lobe, following and surrounding the anterior horn of 

 the lateral ventricle and having for its plan fissure the fissura callosalis : F 

 Plate XXXV, figs. 5 and 6. 



2. The occipitotemporal lobe, following and surrounding the descending horn, 

 and having for its plan fissure the fissura hippocampi : T, Plate XXXV, figs. 

 5 and 6. 



3. The occipital lobe surrounding the posterior horn and having for its plan 

 fissure the fissura calcarina: 0, Plate XXXV, figs. 5 and 6. 



4. The central lobule (Island of Reil) C, Plate XXXV, fig. 5. 



PRIMARY OR TYPICAL FISSURES OF PRIMATE BRAIN. 



1. The fissura Sylvii or fossa of Sylvius. 



2. The mesial arched fissure ; consisting of : — 

 '3. The callosal fissure, surrounding the corpus callosum and following the 



anterior horn. 

 4. The fissura hippocampi, following the fornix and descending horn. 

 .5. The fissura calcarina, following the posterior horn. 



These fissures form the fundamental or plan fissures upon which the convolu- 

 tions of all primate brains are based. They are not only the first to appear in the 

 development of the human brain and in all other Primates, as may be seen by refer- 

 ence to the plates ; but they have also definite relations to the hemisphere itself. We 

 shall also find that all of the other fissures and convolutions, as they appear during 

 the development of the brain, are related in a definite manner to these primary 

 furrows. 





