THE BRAIN IX THE EDENTATA. 305 



If we compare the commissures iu O/'i/cterojms \Yith those of Ilyrinecophacia, we find 

 that the ventral (anterior) commissure is approximately of the same size or even slightly 

 smaller in Ilyrmecophaga, hut the dorsal commissure presents a marked contrast. The 

 fsalterinm is much longer and more markedly attenuated than it is in the African 

 genus, while the corpus callosum is greater, more especially in length, and more nearly 

 horizontal than is the case in Orycteropus. 



The anterior commissure in a representative of the latter genus raeasui'es 4"5 x 

 3'5 mm., and in a specimen of Ilyrmecophaga 3-5x3 mm. ; while the respectiA^e measure- 

 ments of the psalterium are about 12x2 mm. in Orycteropus and about 17 x less than 

 1 mm. in Ilyrmecophaga. The corpus callosum of the latter, measuring about 26 mm. 

 long and with an average thickness of about 2 mm., greatly exceeds in size that of 

 Orycteropus, which is only 21 nmi. long, and about the same tliickness as that of 

 Ilyrm ecoph aga. 



The condition of the commissures in these two geneva points to a most decided 

 superiority in the brain of Ilyrmecophaga, when we recollect that the two brains are 

 approximately equal in size. The anterior (ventral) commissure is larger in Orycteropus 

 because the olfactory bulb and pyriform lobe, from which it is mainly derived, are larger 

 than they are in Ilyrmecophaga. But the larger corpus callosum in the Great Ant-eater 

 points to a marked superiority in pallial development, /. e. a distinctly higher state of 

 cerebral organization, in this animal than in the African Orycteropus. The psalterium 

 of Ilyrmecophaga is more attenuated than that of Orycteropus, partly because it has 

 been subjected to a greater amount of stretching * by the larger corpus callosum, but 

 partly because it is actually smaller in the former, a result of the smaller dimensions of 

 the hippocampus. 



In the cerebral commissures of Tamaiidua we find a considerable family resemblance 

 to Ilyrmecophaga,, making due allowance for the smaller size of the former (fig. 10). 

 The anterior commissure, measuring about 1"5 x 1*25 mm., is placed in a vertical lamina 

 termiualis, which is attached dorsally to the ventral extremity of the psalterium ventrale. 

 The psalterium is elongated and attenuated as it is in Ilyrmecophaga, Ijeing about 9 mm. 

 long, and reaching a maximum thickness of 0'75 mm. ia the plumpest part of the 

 psalterium ventrale ; biit the psalterium dorsale cowsisXa of an extremely thin sheet of 

 crossing fibres. The elongated corpus callosum is more oblique than it is in Ilyrmeco- 

 phaga, and is nearly 14 mm. long and 1 mm. thick. The anterior and posterior 

 extremities of the corpus callosum are distinctly thickened. 



As far as Pouchet's figure allows us to express an opmicm, Cycloturus also presents the 

 typical elongated corpus callosum which seems to be the distinctive character of the 

 family of MyrmecophagicUe among the Edentata. In brains of approximately the same 

 shape the proportion between the lengths of the corpus callosum and the cerebral hemi- 

 sphere may be taken as a rough estimate of the degree of functional perfection of the 

 pallium. Increase in extent of pallium and an increasing richness of texture will find 

 expression in an increased number of crossing fibres. 



* Concerning this stretching inflnence of the corpus callosum, see '.Journal of Anatomy and Physiologj%' 

 vol. sxxii. 1). 41. 



