112 NERVOUS SYSTEM OF MAMMALIA. 



more space, and is placed more on a level with the outer surface 

 of the skull. 



96. Nervous System. The nervous system differs among the 

 various Mammalia, only in the greater or less development of cer- 

 tain of its parts. In all these animals the nervous mass, con- 

 tained in the head, is very considerable, both in proportion to the 

 extent of the body, and relatively to the size of the nerves ; but 

 all the organs which compose it do not equally agree in this de- 

 velopment. Thus, the cerebral hemispheres are very large, whilst 

 the optic tubercles or ganglia are very small, and even nearly 

 rudimentary ; and we shall hereafter see, that amongst Birds, 

 Reptiles, and Fishes, it is quite otherwise. The cerebellum is of 

 considerable size among the greater part of the Mammalia. It 

 always consists of a central lobe of two hemispheres, which have 

 their surface marked by transverse furrows ; and of a commissure, 

 or connecting band, which passes round beneath the spinal mar- 

 row. Moreover, the development of these parts varies much 

 among the different Mammalia, not only as to their size, but also 

 in regard to the channels and convolutions of their surface. For 

 as we pass from Man to the Apes, from these to the Carnivora, 

 and from the Carnivora to the Rodentia, and to the Herbivorous 

 Animals, we generally see the brain become smaller and much 

 smoother. It is also to be noticed that, amongst the Mammalia 

 of the order Marsupialia, the brain presents another character of 

 imperfection, resulting from the absence of the great central com- 

 missure, or corpus callosum, which, in all the other animals of the 

 same class, connects together the two cerebral hemispheres. 

 (ANIM. PHYSIOL. 458.) 



97. Functions of Nutrition. The functions of nutrition are 

 performed among all the Mammalia very nearly as in Man ; 

 hence the structure of the organs, destined for their exercise, 

 varies but little in this great class of animals. It is in the diges- 

 tive apparatus that the most important differences are exhibited. 

 Nearly all the Mammalia are provided with teeth, destined to 

 divide their food ; but, as we have elsewhere seen (ANIM. PHY- 

 SIOL. 182), the number and form of these organs varies accord- 

 ing to the habits of the animal. These organs are sometimes re- 



