THE BEAIN i:*^ THE EDENTATA. 337 



often bent downward, and in Mtjnnecophaga it is quite possible tbat this occasional 

 feature of the Carnivorous brain may occur, for in our type-specimen we find a faint 

 depression y" in the situation where this more extensive sulcus occurs at times in the 

 Dog. 



The only sulcus in the post-Sylvian region of the Dog's l)rain which proseats any 

 constancy is one that Owen calls post-Si/lvian, while Krueg distinguishes it as posterior 

 supru-Syhiun. It corresponds to the sulcus S in Myrmecophaga. But in the Dog it 

 usually forms part of a large arc, the anterior extremity of which may possibly be 

 represented in Myrmecophaga by the small but constant depression S'. 



In the Dog the growth of the Sylvian opercula covers up the Sylvian fossa and forms 

 the fissure of Sylvius, and after that has taken place the tension of further growth is 

 relieved by the formation of the large supra- Sylvian sulcus. 



In the Dog a sulcus, which is not nearly so constant as the posterior supra-Sylvian, is 

 sometimes found between the latter and the Sylvian fissure. Tlie occasional furrow i; in 

 Myrmecophaga may represent this sulcus, which in the Dog is called the "posterior 

 ecto- Sylvian fissure " by Langley {op. cit.), who has modified Wilder's nomenclature. 



The furrow e in Myrmecophaga finds its analogue in a sulcus of the Dog's cerebrum, 

 which "Wilder and others following him have called ecto-lateral. 



From this brief review we liave seen that every sulcus or depression upon the pallium 

 of Myrmecophaga finds an analogue to correspond witli it upon the brain of the Dog. 

 Moreover, we find that if a large number of Dogs' brains be compared, or if we compare 

 the brains of a large number of different Carnivores, the constant and deepest sulci are 

 those which we find represented in tlie brain of the Great Ant-eater. 



There is, however, one important and very significant exception to this generalization. 

 In the Dog, as in all Carnivores, we find a short deep sulcus extending transversely 

 outward from the interhemisphcral cleft near the cephalic extremity of the pallium. 

 This sulcus, commonly known as the crucial, is a very characteristic feature of the brain 

 in Carnivora, and is quite lacking in the brain of Myrmecophaga. This absence is very 

 significant when we recall the fact that physiological experiment has sliown that the 

 pallium which immediately surroimds the sulcus in the Dog is the only " excitable " or 

 " motor " area of the cortex. It may be that the brain of the Dog shows a marked 

 superiority over the brain of the Ant-eater in that the central area, which presides over 

 skUlcd movements, shows a sudden increase in extent, resulting in the formation of a 

 new sulcus, the crucial. 



On the other hand, the formation of the crucial fissure may be to some extent the 

 expression of a general, rather than a local, increase in the extent of the pallium. For 

 we find in the brain of the small Carnivore Geiietta tigrina, which Mivart has described, 

 an absence of a true crucial sulcus, Avhereas all the sulci found in the Edentata are well 

 developed *. In this small and very active Carnivore there are only some shallow pits 

 to represent the crucial sulcus, and, as we have no reason to believe that skilled 

 movements are less developed in this family than in the other Carnivores, it is possible 



* St. George Mivart, " Notes oii some Points iu the Anatomy of the .Elui-oidea,'' Proc. Zool. Soe. London 

 1S82, p. 510, fig. 11. 



