THE BRAIN IN THE EDENTATA. 331 



Tariable, often insignificant or entirely lacking', and altogether of quite minor morplio- 

 logical imjiortance. In the Sloths we have seen that these two elements in the splenial 

 sulcus, the constant element («') and the varial)le element (a"), ai'e quite distinct the one 

 from the other. 



If we turn from the consideration of the splenial sulcus in the majority of mammals to 

 its condition in the Primates, we are surprised to find that the sulcus which is usually 

 described as the " splenial " is lacking in its most essential part, namely in that retro- 

 splenial j^ortion which we have just learned to regard as the only constant part of the 

 sulcus. Thus Tm-ner *, in common with many other writers, uses the term " splenial 

 fissure "as synonymous with " calloso-marginal fissure," tacitly implying a homology 

 which is extremely questionable. We are quite prepared to admit that the calloso- 

 marginal fissure of Man and the other Primates may represent the inconstant and mor- 

 phologically unimportant {a") element of the splenial sulcus, but it obviously doas not 

 represent the whole, nor in fact the most important part, of that fissure. Moreover, it 

 seems highly improbable that the most constant sulcus of the pallium throughout the 

 Mammalia, the earliest fissure to make its appearance in development and often the only 

 " complete " sulcvis present, should entirely vanish in the Primates. 



One of the first, if not actually the earliest, sulci to make its appearance in the 

 development of the human pallium is a short, oblique, and very deep farrow a short 

 distance behind and slightly below the splenium of the corpus callosum. This important 

 sulcus has been called bv Professor Cumiin2:bani f the " anterior calcarine.'" The features 

 of this sulcus in its earliest stage are so analogous to the earliest stage of the " splenial 

 fissure " of Krueg in the developing brain in the Cat that the question of their homology 

 naturally suggests itself. When we remember that the anterior calcarine sulcus is a 

 complete and a very constant sulcus, of obviously great morphological importance, and 

 that it makes its appearance in a situation analogous to that occupied in most mammals 

 other than the Primates by a very constant sulcus Avhich is sometimes complete and of 

 undoubted morphological importance, there is sufficient evidence to justify us in 

 suggesting, as a tentative working hypothesis, the morphological identity of the two 

 structures, i. e. of the anterior calcarine fissure of Primates with the essential or caudal 

 part of the s]jJ en ial fissure of most other mammals. Por it seems to me unlikely that such 

 a deeply-imprinted feature, which, almost alone of all the pallial fissures, tends to make 

 its appearance in all the mammalian orders, except the specialized Monotremes, should 

 suddenly disappear in the Primates without leaving any trace behind. If we glance at 

 such a brain as that of Lemur nigrifrons, which Turner has reproduced % from a memoir 

 of Plower, the resemblance of the calcarine fissure to the a' element of the splenial sulcus 

 is indeed striking, since the sulcus in question presents the characteristic relationship to 

 the rhinal fissure. In Man and the Man-like Apes the similarity is not so striking, not 



* Turner, o^;. dl., Journal of Anatomy and Ph) siolog3% vol. xxv. p. 143. 



t D. J. Cunningham, " The Surface Anatomy of the Primate Cerebrum," Cunningham Memoirs of the Royal 

 Irish Academy, No. vii., 1892. 



% Turner, op. a'(., Journal of Anatomy and Physiology, vol. xxv. fig. 38. 



