212 ORIGINAL ARTICLES. 



on its'right side a well-marked, superior bridging convolution, coming 

 for a considerable part of its length nearly or quite to a level with the 

 lobes it connects. Tiedemann's figure of the chimpanzee's brain leads 

 us, by its imperfectly-marked operculum, to the same conclusion as its 

 sharply drawn one did in the case of the orang. The law of correlation 

 of forms is a safe guide to us, when we have to predict what will be 

 found in the lower organisms of well-marked families ; it loses its in- 

 flexibility, and becomes but a leaden rule, when we come to examine the 

 most perfectly evolved species in such families. In the higher species 

 of the order, apes, as in the higher varieties of the species, man, we find 

 variability the rule, uniformity the exception ; in the lower species, as in 

 the lower varieties of man, the reverse condition obtains. The variabi- 

 lity which we have seen to exist in the species chimpanzee, is no incon- 

 siderable proof of its high relative rank in its own order. 



But there is a second connecting bridge passing between the occipi- 

 tal and the parietal lobes. This convolution is invariably present, and 

 invariably superficially placed in man ; it is as invariably absent in both 

 the anthropoid apes. In man it is always a large, easily recognizable 

 structure; and in cases such as those which our fourth human brain may 

 be taken to exemplify, or exaggerate, it will often be found to send a 

 branch, as it were, in aid of the weakened superior bridge. The vacuity 

 which in the apes corresponds to what is invariably a convolution of 

 importance in man, may be seen in fig. 1, immediately posteriorly to 

 6 ; and in fig. 3, immediately below a. But this convolution, the 

 " deuxieme pli de passage" of Gratiolet, absent without exception in 

 the Old World apes, and present equally invariably in man, is found also 

 in two New World monkeys, the cebus capucinus* possessing it with- 

 out, the ateles possessing it in company with its fellow, f 



There is yet a third structure — " the Lobule of the Marginal 

 Convolution" — to be treated of. In man it lies above the upper 

 end of the fissure of Sylvius ; and it may not unfairly be repre- 

 sented in our figure 1, by the convolution which lies immediately to 

 the spectator's left of 5. Of it M. Gratiolet speaks in the following 

 language : — " Cet lobule est particulier a l'homme et ne se trouve pas \ 

 ni dans l'orang ni dans le chimpanzee." But I find nowhere in M. 

 Gratiolet' s work any repetition of this striking statement : indeed it loses 

 a good deal of its force, when we find the qualifying words " souvent 

 assez grand" applied to this peculiarly anthropic lobule in the sentence 

 immediately preceding the one we have quoted. And in the coloured 

 diagrams, which speak so plainly, by their various hues, of the varied 

 relations in extent and arrangement which may obtain among different 

 brains, I find no separate colour assigned to this peculiarly separable 

 lobe — no such distinction is awarded to them as there is to the bridging 

 "plis de passage;" which, nevertheless, are not asserted to be exclu- 



Gratiolet, Memoire, p. 78. f Ibid., p. 76. % Ibid -> P- 60 - 



