334 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



of the occipital lobe into the second or middle occipitotemporal convolution. It 

 separates the fissura occipitalis prima or upper branch of the primary occipital arch, 

 O 1 , from the lower branch, the fissura occipitalis secunda, O 2 . The fourth, Y, lies 

 below this and passes into the third temporal convolution. Bischoff has asserted 1 that 

 the superior external and the superior internal pli de passage are identical. As 

 regards this observation I have found that in some monkey brains they together 

 form a single gyrus, whilst in others two can certainly be distinguished; so that to a 

 certain extent the statements of Gratiolet, Ecker, the English anatomists and 

 some others must be modified. 



Ecker, opposing Bischoff 's interpretation, remarks 2 as follows : "Bischoff is of 

 the opinion that this convolution (superior internal pli de passage} is homolo- 

 gous with the first outer transition convolution of Gratiolet, our gyrus occipitalis 

 primus and hence is wanting where the latter is developed, and vice versa. I 

 regret to be obliged to oppose this view. Not only do we find, as I will more 

 particularly describe in another place, in the brain of various apes [Cercopithecus, 

 Cynocephahis, etc.) both convolutions most clearly developed together, but we also 

 not infrequently find in Man a convolution which, arising at the posterior extremity 

 of the precuneus with the gyrus occipitalis primus (Ecker), runs backward in 

 an arch convex inward and downward, while the former (gyrus occipitalis pri- 

 mus, Ecker), as is known, makes an arch outward. In the cuneus the two convo- 

 lutions again coalesce. This convolution is regularly sunk into the depths of 

 the fissura parieto-occipitalis, and only becomes visible on pulling apart the bor- 

 ders of this fissure ; but sometimes it also comes to the surface and causes there 

 a very unusual appearance of this part of the cerebral surface." 



I think I shall be able to show conclusively, that Bischoff is at least thus 

 far correct in his observation and identification that in some monkey brains the 

 condition as he has described it exists, whilst in others, the conclusions which 

 Ecker draws from his experience represent the true condition of affairs. This 

 will show the reason for their disagreement and at the same time will completely 

 reconcile the two opposed opinions as held by them. A glance at the mesial 

 surface of Macacus nemestrinus, Plate XXXVII, fig. 5, will render this clear. 

 An inverted S-shaped convolution, 2, will be seen passing from the precuneus, P.O., 

 backward to join the occipital arch. The anterior arch, a, of this inverted S-shaped 

 gyrus is directed downward and inward, whilst the posterior arch, b, is directed 

 upward and outward. 



Ecker has apparently not met with this condition, where evidently a and b 

 form together a single gyrus as is so plainly indicated in the figure. They are, 

 however, not always so plainly differentiated as in this specimen of Macacus 

 nemestrinus, and a comparison of many monkey brains of different genera and 

 species has enabled me to understand the objection of Ecker to Bischoff 's obser- 



^ie Grosshirnwindungen des Menschen, etc. 



2 Cerebral Convolutions of Man, Edes transl. Note, p. 75. 



