20 ON THE AFFINITIES OF THE BRAIN 



measure of the position ' of the third cornu l ; indeed, when we find 

 Tiedemann speaking of the ' pedes hippocampi minores ' as ' pro- 

 cessus 2 duo medullares qui a posteriore corporis callosi margine 

 proficiscuntur,' it is easy to understand how he came to overlook 

 their existence altogether, * in cerebro Simiarum desunt,' being so 

 far in error as to their relations to neighbouring parts. 



The relation of the posterior edge of the corpus callosum to the 

 commencement of the third cornu is of importance, not merely as a 

 guide to the discovery of that fissure, but also as, when coupled 

 with the relations which the corpus callosum holds to the internal 

 occipital fissure, 16, laterally, and to the corpora quadrigemina 

 posteriorly, speaking unambiguously of great diminution of the 

 antero-posterior diameter of the simious corpus callosum. 



The hippocampus major presents several well-marked corrugations 

 on the expanded lower extremity, ' quae huic parti tanquam figuram 

 digitorum pedis tribuunt 3 ;' but as they are on its posterior, not on 

 its anterior edge, we are not compelled to contradict Tiedemann's 

 twenty-first corollary, which relates to the hippocampus major, in 

 the same way as our figures compel us to differ from his twenty- 

 second, quoted above, with reference to the hippocampus minor. 



It is for the sake of illustrating yet further the important 

 principle, that variability of arrangement is to be expected, rather 

 than wondered at, in organisms as high as those of these apes, that 

 I add the following observations as to the convolutions on the 

 internal aspect of the hemispheres. There is scarcely any indica- 

 tion of a lobulus quadratus 4 , the structure representing it resembles 

 but little the figure of it as given by M. Gratiolet in his third 

 plate, whilst, as if in compensation, the superior marginal convolu- 

 tion, spoken of by him as 'tres simple et a peu pres lisse dans 

 l'orang ' (page 49 in his Memoire), presents, in our specimen, abun- 

 dant and rich convolutions. 



1 'Nat. Hist. Review,' 1. c, p. 79. 



2 'Icones,' p. 51. 



3 'Icones,' p. 51. But in a work or paper published 1826, Tiedemann acknowledges 

 the existence of the digitiform process in the orang. Cf. St. Hilaire, ' Hist. Nat.' 

 ii. p. 221. 



* This quadrilateral lobule is spoken of in the ' Anatomie Comparee/ Tom. ii. 1857, 

 p. 115, as ' un petit lobule quadrilateral, que de petites incisures subdivisent.' This 

 I had not seen when I wrote as above. P. 45, ' II n'existe jamais dans les singes 

 et ne se voit pas toujours dans l'homme.' Cp. Wagner, ' Vorstud.' ii. pp. 32, 33, where 

 it is said to be present ' bei der Mehrzahl der Menschen.' 



