302 DR. a. ELLIOT SMITH ON 



are proceeding from that essentially mammalian cortical area, the pallium. In all 

 mammals except the Monotremata and Marsupialia (in which the pallial fibres cross the 

 mesial plane in the veiitral commissurel a large and increasing proportion of these pallial 

 fibres invade the dorsal commissure nnd form the corpus callosum. When we remember 

 that this structure has been acquired in its jii-esent form within the mammalian class, 

 we naturally turn with great interest to the examination of the commissiues in such an 

 enigmatical order of mammals as the Edentata. 



Pouchet says * that the Edentates, considered as a whole, present a remarkable 

 diminution in the size of the corpus callosum. He says that in a young Choloejjns, which 

 was 19 cm. long, the corpus callosum was 7"5 mm. long and 0'5 mm. thick. la the 

 specimen of Cholcppns Hoffmanni which Turner figures t the corpus callosum is repre- 

 sented as being 11 mm. long and 1 mm. thick in a hemisphere whose maximum length 

 (without the olfactory liulb) is 8G mm. 



In Elower's memoir there is an enlarged figure of the mesial surface of the brain of a 

 Choloppus d/df/cti/lns J, but unfortiinately the degree of magnification is not indicated. In 

 a hemisphere 70 mm. long, he represents a corpus callosum 23 mm. long and 2-8 mm. 

 thick. Judging fi-om the average dimensions of the adiilt hemisphere of Cholcejms, these 

 measurements are probably about twice the actual size of the objects. In my specimen 

 of Cholcepns didactyhis (which had been in alcohol for some years) the cerebral hemi- 

 sphere is barely 35 mm. long, and the corpus callosum measures 10'75 mm. loug and 



I mm. thick. 



We may safely conclude that the corpus callosum in the adult Two-toed Sloth is about 



II mm. long and about 1 mm. thick. We cannot speak with the same degree of certainty 

 of the measurements in Bradyptis, because there is no information upon the subject in 

 the past records, and my only specimen available for measurement is not fully grown. 

 In the bi-ain of Bradypus tridactyliis which Professor Max Weber generously gave me, 

 the cerebral hemisphere is 26 mm. long, and possesses a corpus callosum which is 6"75 

 mm. long, and 1 mm. thick. In the adult brain the cerebral hemisphere attains a length 

 of about 32 mm. In this young Three-toed Slotli we have a corpus callosum which is 

 much shorter, both absolutely and relatively (to the length of the hemisphere), than is the 

 case in the Two-toed Sloth, but this shortness is compensated by a greater thickness, for in 

 this small brain of Bi'adypus the corpus callosum has already attained to the same 

 thickness which it readies in the much larger brain of the adult Clwloepns. 



Poiicbet §, after describing the corpus callosum in tlie Sloths, proceeds to descril^e 

 that of the Ant-eaters. He says that in Cycloiurm aud Myrmecophaga it presents similar 

 features to that of the Sloths. 



Tliis is a very loose and utterly misleading statement, for, as we shall see subsequently, 

 there is a very marked contrast between the features presented by the two families in 

 this respect. He gives the measurements of the corpus callosum in Myrmecophaga as 



* Op. at., Journal de rAuatomie et de la Physiol, tm. vi. p. 308 et seq. 



t Turner, o^<. cit., Journal of Anatomy and Physiology, vol. xxv. fig. 15. 



X Flower, 0^1. cit., Pbil. Trans. 18(55, pi. xxxvii. fig. 5. 



§ Oj,. at. p. 308.; 



