238 ZOOLOGY. 



of two hemispheres, and of a commissure, surrounding the me- 

 dulla s])iii;dis superiorly, the annular protuber an <> Men- 

 over, many differences exist among mammals in respect of 

 these orpins, as well as regards the depressions and circum- 

 volutions on tin- surface tit the brain. As we descend from 

 man to apes, from these to the carnivora, and from the car- 

 nivora to rodents, the brain generally becomes smaller and 

 smaller, and smoother. The face also is developed in an 

 inverse ratio to that of the brain, and the measure of the 

 intellectual faculties may be guessed at by observing this 

 increasing size of the face as compared with the brain 



In the marsupialia and monotremes, the brain seems still 

 further degraded by the absence of, or at least the rudimen- 

 tary state of, the corpus callosum, which in all other mam- 

 mals unites the hemispheres of the brain to each other. 



403. Functions of Nutrition. The functions of nutri- 

 tion resemble each other throughout nearly the whole of the 

 class ; the digestive tube is the organ which shows the most 

 remarkable variety in its arrangements. 



Nearly all have teeth ( 53), which vary in number and 

 form, in accordance with the kind of food they live on. But 

 they may be replaced by whalebone, as in certain of the 

 cetacea (Figs. 13 and 14), or by a horny bill, as in the onii- 

 thorhynchus paradoxus ; hence the name. 



404. The stomach is usually simple, as in man (Fig. 24) 

 and the ape (Fig. 4) ; but, as in ruminants, the organ may 

 be subdivided into a number of compartments, from the first 

 of which the food, after remaining a certain time there, re- 

 turns to the mouth by the gullet, to be remasticated (rumi- 

 nation), and thence transmitted into the following compart- 

 ments, without again passing into the first. 



These stomachs in ruminants are four in number. The 

 first, which is the largest, is called the paunch (Fig. l*7i: 

 its surface is furnished with papillae and an epidermic cover- 

 in- (Fig. 188); it fills a large part of the left side of the 

 abdominal cavity. The second stomach, called the kind's 

 hood, is small, and looks like an appendage of the paunch. 

 But it differs internally (Fig. 188), being composed of a 

 number of folds disposed so as to form polygonal cells or 

 cellules, like honeyosmb. The third stomach, not so small as 

 the second, is called maniplus, from the number of longitu- 

 dinal folds seen in its interior; finally, the fourth is called the 

 rennet ; its property of curdling milk renders it remarkable. 

 The first, second, and third stomachs communicate directly 



