MAMMALIA 67 



typical condition, though all sorts of reductions in number may 

 take place, by which a fitness for special kinds of locomotion, &c., 

 is brought about. The most extreme case is that of the horse, 

 which possesses but one well-developed digit in each limb. 



Digestive Organs (see pp. 34-38). In most respects the 

 digestive organs of Man may be taken as typical, especially as 

 regards the fact that there are two sets of teeth, and that these 

 teeth are of several kinds. The total of thirty-two for the per- 

 manent set is not, however, typical, a much nearer approach to 

 this being found in the pig, which possesses forty-four teeth in 

 all, arranged according to the following dental formula: 



Many Mammals, however, have undergone much greater re- 

 duction in the number of teeth than human beings, and in some 

 species these structures have been lost altogether, as in the Great 

 Ant-eater of South America. Porpoises and the like, on the other 

 hand, have a very large number of teeth not divided into distinct 

 kinds. The nature of the teeth, which is determined by the kind 

 of food, is of great classificatory importance. 



Circulatory Organs (see pp. 38-43). Hot blood (about 98 F.), 

 a four-chambered heart, with impure blood on its right and pure 

 blood on its left side, and an aorta with its arch curving to the left, 

 are the most characteristic features, and have all been dealt with 

 in the preliminary sketch. 



The necessity of maintaining the blood at a temperature often 

 higher than that of the surrounding medium appears to be one 

 reason for the possession of a thick coating of hair. 



Respiratory Organs (see pp. 45-47). No Mammal at any time 

 possesses gills, though gill-slits are found in the embryo (see p. 62), 

 but breathing is always effected by a pair of spongy lungs situated 

 in the cavity of the thorax. There is present, as in Man, a partly 

 muscular partition, the midriff or diaphragm, separating the cavity 

 of the thorax from that of the abdomen, and materially assisting 

 in the movements of breathing by which the air in the lungs is 

 renewed. 



Nervous System (see pp. 4953). The arrangements described 

 in Man may be taken as fairly typical for Mammals in general, 

 except as regards the brain, which is proportionately very much 

 larger than in any other form. 



