294 



MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



tal lobe directly back of this fissure (occipitalis externa) has been called by Gra- 

 tiolet the operculum, because in the monkeys, and sometimes, though rarely, 

 in Man, it covers in or conceals two small convolutions running from the occipital to 

 the parietal lobe : the superior external or premier pli.de passage and the deuxieme 

 pli de passage exteme. Near the lower border of the occipital lobe there is found 

 another fissure, O 2 , Plate XXXVII, figs. 1, 2, 6, 7, 8, 9 ; Plate XXXVIII, figs. 4, 11, 

 the two together, O 1 and O 2 , cutting off" a posterior triangular portion, the apex 

 looking forward, which maps off distinctly, upon the lateral surface, the outlines 

 of the future occipital lobe. As the exact significance of these two fissures taken 

 together has not been noted by previous investigators, I propose for them the 

 name primary occipital arch, the relations of which externally may be studied on 

 PI. XXXVII, figs. 1 and 19. The upper branch, O 1 , corresponds with the com- 

 bined fissura-perpendicularis interna and externa, but in order to designate it as a 

 whole and to render its important character distinct I propose for it the name, 

 superior occipital fissure (fissura occipitalis superior). The lower branch, O 2 , 

 curiously enough, has received little attention, and in Man it has been seldom even 

 recognized as a distinct fissure except by Wernicke and a few others. It has some- 

 times been designated in the nomenclature of the human brain as Wernicke's 

 fissure. I propose to call it the inferior occipital fissure, (fissura occipitalis inferior). 

 These two fissures of the arch pass forward toward the apex of the occipital lobe 

 where they are separated from each other by a small bridging convolution, P, one 

 of the pits de passage of Gratiolet. Posteriorly they pass around on to the mesial 

 surface and their extremities are related in a scroll-like manner to the ends of 



the transverse calcarine fissure, t. ca., 

 as represented in fig. 6, which should be 

 compared with Plates XXXVII, fig. 5 and 

 XXXVIII, figs. 5 and 12. It will be seen 

 at once that the primary occipital arch, a 

 name I have given it on account of these 

 relations, cuts off a perfectly symmetrical 

 occipital lobe which is also related, as we 

 shall see hereafter, to the occipitofrontal 

 and occipitotemporal lobes in a perfectly 

 definite and symmetrical manner. 



In studying the temporary fissures of the 

 mesial surface, fig. 2 and PI. XXXV, fig. 

 4, we found two fissures cutting off a rhomboidal shaped mass, O 1 , O 2 . Of these, 

 the lower one completely disappears, whilst the upper one, O 1 , partly remains to 

 form a portion of the permanent fissure. I believe, at least, that I have been able 

 to determine that such is the case, and in fig. 4, PL XXXV, I think it can be dis- 

 tinguished as a small oblique fissure situated at the end of, and communicating 

 with the calcarine. Ecker also states that he believes he has been able to make 

 this out, but is not positive about it. However this may be, what I particularly 



Fig. 6. 



