THE BRAIX I^" THE EDENTATA. 281 



the only attempt to treat in a comprehensive manner the anatomy of the brain in the 

 whole group. But Pouchet's memoir is altogether inadequate to our present needs. 

 During tlie thirty years that have elapsed since this work was published the outlook of 

 the student of brain anatomy has been immeasurably widened, his methods of investi- 

 gation have been utterly changed, and his whole science has been revolutionized. The 

 investigator of to-day approaches the study of a new type of brain with a much better 

 equipment and a considerably clearer appreciation of the significance and relative 

 importance of anatomical features than was possible before Weigert, Golgi, and Marclii 

 introduced methods of research which have enabled us to understand the fundamental 

 plan of the organ with which we have to deal. Instead of describing in a more or less 

 casual manner the general features of a brain as Pouchet did, and thereby accumulating 

 a mass of evidence which is valueless to a succeeding generation, we can, in the light of 

 our present-day knowledge, directly examine with a clearly defined purpose those parts 

 of the brain which we know are undergoing change in the Mammalia. Of all parts of 

 the brain the most recently differentiated portion, the cerebral cortex, is naturally that 

 which is the most changeable, and to which most of our attention must necessarily 

 be devoted. In studying this more than any other part it is necessary to institute 

 comparisons with as wide a series of mammals as possible, in order to gain an insight 

 into the significance of such modifications as we find. 



In the light of our knowledge of. the important differences which exist between the 

 hippocampal formation and the cerebral commissures in the Monotremata and Marsupialia, 

 and between each of these orders and the so-called Placentalia, I have entered into the 

 consideration of this aspect of the problem in some detail. The regions of the brain- 

 stem, the optic thalamus, the mesencephalon and hind-brain, have been described as briefly 

 as is consistent with clearness, because these stable regions of the brain are subject 

 to very slight variations as compared M'ith the great modifications which affect other 

 regions. 



In considering the cerebellum I have entered into the descriptions in considerable 

 detail because, unfortvmately, the subject of cerebellar morphology is almost a term 

 incognita. For the purposes of this paper I have found it necessary to examine the 

 cerebellum in as many mammalian brains as were obtainable, in order to arrive at some 

 idea of the fundamental plan according to which this complex organ was built up. As 

 the result of this extensive examination I have elaborated a tentative scheme for 

 describing the cerebellum, the chief merit of which is its extreme simplicity, and the 

 fact that it is applicable to the cerebellum in any member of the Metatheria or 

 Eutheria. This can hardly be said of the method of description which is employed in 

 human anatomy, and which is practically the only system of nomenclature thus far 

 adopted in discussing the cerebellum. 



In certain of the better known regions of the brain I have found it necessary to enter 

 into much longer explanations than might otherwise have been needed if the nomenclature 

 liad not been so confused. As an examjile I mtiy refer to the region of the pyriform lobe. 

 ^J.So far as was possible with the material at my disposal I have investigated the 

 histology of the brains considered in this contribution, and in the case of Chlamydophorus 



