THE BEAIiSr OF MAMMALS AND THE OLFACTORY APPAKATUS. 309 



comitant. Man has, in this respect, reached a stage where many of the functions con- 

 cerned can no longer be executed without the participation of the cortex. All the 

 possible transition-stages are observed in mammals. It is true that the separate 

 muscles, etc., can be influenced in mammals through irritation of the cortex, but the 

 parts of the cortex thus involved are not necessary for the movements concerned. 

 In man, however, the greater portion of the surface of the forebrain has become in- 

 dispensable. 



Morphologically, this relation expresses itself in a very different develop- 

 ment of the various parts of the brain-mantle. At present the essential parts 

 of the mantle can be distinguished from one another in a few mammals only; 

 yet it is already known that the development of the cortex is continuous in the 

 mammalian series. There exists the greatest of yariations and the most 

 variable of size-relations; yet even at present the position which a few of 

 the mammals occupy in the entire series can be indicated. Even a super- 

 ficial consideration of mammalian brains shows that one center especially, 

 the olfactory center, presents most varying size-relations, so considerable, 



Fig. 139. — Brain of an armadillo: Dasypus riUosiis. (Side-view.) 

 The olfactory apparatus is shaded. 



sometimes, that the entire remaining portion of the mantle appears to be a 

 small appendage only of the olfactory lobe. 



The olfactory brain is that part of the cortex which first appears in the 

 animal series; the other cortical regions become only later associated with 

 this. Among many of the lower mammals animals are known which possess 

 rudiments, at first only relatively small, of that portion of the mantle which 

 does not belong to the olfactory apparatus. In such a case, the olfactory 

 lobe, and whatever else belongs to the cortical olfactory apparatus, often 

 forms almost one-half of the entire mass of the forebrain. 



What is known regarding the manner of life of such ''olfactory'' animals agrees 

 well with the structure of their brains. The small armadillo, for example, the brain 

 of which is represented above, spends its entire life burrowing in the soil and creep- 

 ing about under the foliage of the dense priijieval forests. For choosing its food, for 

 finding it, no sensory apparatus will be so important to it as that of smell. The 

 uniformly limited activities of the plump body will need fewer acquired and de- 

 liberate acts than the prehensile hand of an ape, perhaps. In the latter, therefore, 

 we should expect a much greated development of the true psychical centers for the 

 upper extremities than in the small creature that lives by wallowing. This, in fact. 



