NERVOUS SYSTEM. VERTEBRATA. 145 



Family ECHIDNID^E. 



D. 191. The brain of a Spiny Anteater (Tacliyglossw [Echidna] 

 aculcatus), in which the greater part of the lateral and 

 dorsal walls of the left cerebral hemisphere have been dis- 

 sected away so as to expose the hippocampus in the lateral 

 ventricle. 



In spite of the marked contrast between the gen oral 

 appearance of this brain and that of the Platypus, there is 

 the closest structural agreement in most of the essential 

 features of the two. 



The most obtrusive feature of this brain is the relative! v 

 enormous development of the cerebral hemispheres, which 

 are much larger, both actually and relatively, than those 

 of the Platypus. In addition the extent of the cortex is 

 very considerably increased by numerous deep sulci. The 

 meaning of this large neopallium is quite incomprehensible. 

 The factors which the study of other mammalian brain-; 

 has shown to be the determinants of the extent of the 

 cortex, fail completely to explain how it is that a small 

 animal of the lowliest status in the mammalian series comes 

 to possess this large cortical apparatus. In other small, 

 terrestrial, insect-eating mammals such as the Pangolins 

 and the Anteaters, and in the fossorial Bandicoots, Hedge- 

 hogs, and Armadillos, we find highly macrosmatic brains 

 with small neopallia : and yet in Tachyylossus, whose mode 

 of life is not dissimilar to many of these mammals, we 

 find alongside the large olfactory bulb and great pyriform 

 lobe of the highly macrosmatic brain a huge complicated 

 neopallinm. 



In the remarks concerning the brain of the Platypus 

 (D. 189), it was tentatively suggested that the enormous 

 development of the trigeminal nerves might explain to some 

 extent the large size of the receptive organ and "storehouse," 

 so to speak, for tactile impressions. But such a suggestion 

 does not help us in the case of the Spiny Anteater, because 

 the trigeminal nerves are much smaller than they are in the 

 Platypus, and yet the neopallium is much larger. The 

 eyes and optic tracts are still very small, as in the Platypus, 

 so that another factor in determining a large pallial area is 

 lacking. 



VOL. II. L 



