MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS 335 



vations, and how in some eases this gyrus, 2, 2\ lias been described, sometimes 

 as one and sometimes as two distinct convolutions. In some specimens of Ma- 

 cacus cyuomolgus and M. nemestrinus, I have been able to note this convolution 

 as single whilst in other cases in the same species the part of the convolution 

 marked x was not completely separated from the cuneus, thus giving rise to the 

 appearance of two separate convolutions (compare fig. 14, Plate XXXYII. 

 with %. 12, Plate XXXVIII, and fig. 5, Plate XXXVII), a superior internal pli de 

 passage, 2', and a superior external pit de passage, 2. In some this connection 

 with the cuneus was very slight and in others better developed, and a perfect 

 series could be traced as we advanced from the lower to the higher apes in which, 

 in some cases, the tendency was to differentiate into a single gyrus and in others 

 two distinct gyri were found, as in fig. 14, Plate XXXVII, where the superior ex- 

 ternal pit de passage lies concealed within the occipital cleft and the superior inter- 

 nal is shown extending between the cuneus and precuneus, 2'. 



The condition in Cebus apella, Plate XXXVIII. fig. 12. is identical with that 

 found in Macacus nemestrimts, Plate XXXVII, fig. 5, except that in the latter 

 brain, as drawn, the occipital cleft has been widely expanded. A more highly 

 developed condition is seen in Plate XXXVII, fig. 7. The singleness of the 

 convolution is, however, still evident. Ecker distinguishes the anterior arch, a, 

 Plate XXXVII, fig. 5, as Gratiolet and others have done, as a distinct gyrus, 

 which it often is, (the superior internal//? ' de passage), and the outwardly directed 

 or posterior arch, b, as another distinct and separate gyrus, which he calls the 

 gyrus occipitalis primus, and which Gratiolet and other writers have described 

 also separately under the name of the superior external pit de passage, the annect- 

 ant, bridging or connecting convolution of the English anatomists Rolleston, Turner, 

 Marshall and Huxley. In some cases at least, these two separate gyri differen- 

 tiate or conjoin to form a single convolution corresponding with the observations 

 and description as given by Bischoff and as illustrated on Plate XXXVII, figs. 5, 7 

 and 14 and Plate XXXVIII, fig. 12. 



In Ateles, PI. XXXVII, figs. 15 and 17, the separation of the two arches is 

 pushed still further, for in this brain the anterior arch of the gyrus is concealed within 

 the fissure, O 1 , whilst the posterior makes its appearance on the lateral surface of the 

 brain as the convolution, 2, thus separating the fissure O 1 into two portions, as is the 

 case in Man, the Chimpanzee, etc. 



The following may, therefore, be considered as representing the exact state of 

 the case : there are usually in this region two separate and distinct gyri as claimed 

 by Ecker, Gratiolet and others but in some cases but a single convolution can be 

 found, which represents, however, in its arches the combined superior external and 

 internal pits de passage. 



The interior internal pli de passage is well developed in the Simiadae. It passes 

 from the apex of the cuneus forward and joins the convolution M.O.F., the mesial 

 occipitofrontal (gyrus fornicatus). It may be seen in the figures marked 1, and is the 

 gyrus which in the Simiadae separates the mesial portion of the first occipital or 



42 JOURX. A. X. S. PHILA., VOL. X. 



