284 VERTEBRATE ANIMALS. 



.22 11 22 33 



In this way the dentition that is to say, the number and ar- 

 rangement of the teeth can be presented in a manner which 

 can be instantly recognized by the eye. It must be remem- 

 bered, however, that the formula seldom exhibits the regular- 

 ity of the one of the Chimpanzee given above. The teeth 

 are not necessarily the same in both jaws, and in many cases 

 some may be altogether wanting. To show this there is sub- 

 joined the dental formula of a typical Ruminant animal, such 

 as a sheep : 



.00 00 33 33 



33 11 3=3 



From this formula it will be seen that the sheep has 32 teeth 

 in both jaws taken together. The upper incisors and canines 

 are wanting, and there are three praemolars and three molars 

 on each side of the upper jaw. In the lower jaw there are 

 six incisors, two canines, and the same number of prasmolars 

 and molars as in the upper jaw. 



As regards the digestive system of Mammals, the alimen- 

 tary canal and digestive glands have on the whole the same 

 general structure and arrangement as in man (pp. 203, 204). 

 Some very remarkable modifications, however, in the structure 

 of the stomach and in the termination of the intestine occur 

 in certain Mammals ; but these will be noticed in speaking of 

 the orders in which they occur. 



The cavity of the abdomen in Mammals is always separated 

 from that of the thorax by a complete muscular partition the 

 diaphragm. The abdomen contains the greater part of the 

 alimentary canal, the liver, pancreas, kidneys, and other or- 

 gans. The thorax contains chiefly the heart and lungs. The 

 heart is contained in a membranous sac the pericardium, and 

 consists of two auricles and two ventricles. The heart con- 

 sists functionally of two sides, each having an auricle and a 

 ventricle, which communicate with one another by apertures, 

 so guarded by valves that the blood can pass from the auricle 

 into the ventricle, but not, under ordinary circumstances, from 

 the ventricle to the auricle. There is in the adult no direct 

 communication between the two sides of the heart. The 

 course of the circulation is indicated in the subjoined diagram, 

 and is shortly as follows : The venous blood, which has become 

 impure by passing through the tissues, is returned by the 

 great veins to the right auricle, from which it passes into the 



