306 



DE. G. ELLIOT SMJTIl UN 



In Myrmecophaga the leugth of the corpus callosum is about 48 per cent, of the length 

 of the hemisjihere ; in Tamaudna it is about 41 per cent., and in Cycloturus it is about 

 38 per cent. 



In Orycteropus, an animal of similar habits , and approximately equal size to 

 Myrmecophaga, the corpus callosum, which is about the same thickness as it is in 

 Myrmecoplwga, is only 35 per cent, of the length of the hemisphere. 



The cerebral commissures in the Sloths present a marked contrast both in appearance 

 and in size to those of their relatives — the American Ant-eaters. We have just seen that 

 the characteristic feature of the corpxis callosum in the latter is its great length ; the 

 effect of this elongation is most noticeable in the caudal direction, and as the upper 

 extremity of the psalterium is attached to this extremity of the corpus callosum it follows 

 that the psalterkmi dorsale becomes markedly attenuated. 



Fig. 17. 



psalteriam 

 hippocamp. vestigia 



bulb, olfaot. ' 

 area praecommiss. 



tubercul. olfact 



Corp. oallos. 



tela 



fisB. prima 



/ Corp. qaadrigem. 



aquaeduct, Syl. 

 vel. medallare 



'\ pone Varol. 

 lob. 

 pyriform. ooramisa. moll. 



oommiss. ant. / 

 tract, opt'. 



ventr. III. 



Brain of Bradypvs tridactylua, jiiv. ; surface exposed by mesial sagittal secli 



Tela refers to roof of third ventricle. 



Enlarged 2 diam. 



In the Bradypodid(B we find a vei'v short corpus callosum ; in three specimens of 

 Cholcepus tlie average length is 30 per cent, of the length of the hemisphere ; and in a 

 young specimen of Bradypus the j)roportion is as low as 26 per cent. These proportions 

 are all the more noteworthy if we recall that the hemisphere in the Sloths is itself 

 relatively short compared with that of the Ant-eaters. 



The shortness of the corj)us callosum explains the peculiar shape of the psalterium. 

 For, being free from the stretching influence of a caudally elongating corpus callosum, 

 the psalterium remains as a small plump mass of fibres, with no division into p)saUeritmi 

 dorsale and ventnde. Tiiis little mass of fibres is thickest at its junction with the corjjus 

 callosum, and rapidly tapers to a rounded point as it extends downward and forward, 

 forming an angle of about 30° with the corpus callosum. It is about half the length 

 of the latter. Flower soys * that "the anterior end [of the corpus callosum] is simple 



Flciwer. Phil. Trans. 18G.5, ]). 630. 



