310 VERTEBRATE ANIMALS. 
in the upper row referring to the upper jaw, and those in the 
lower row to the lower jaw ; the short dashes between the figures 
of each row still indicating the teeth on the two sides of the 
mouth. The complete formula would therefore run as follows :— 
i ats — $ pm —— a m a = 32: 
In this way the dezzztcon—that is to say, the number and ar- 
rangement of the teeth—can be presented in a manner which 
can be instantly recognised by the eye. It must be remembered; 
however, that the formula seldom exhibits the regularity 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 subjoined the 
dental formula of a ales Ruminant animal, such as a sheep :— 
Z 
ier a) mre mm 2 3 = 32, 
37=3 I—I oe aa 
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 preemolars and three molars on 
each side of the upper jaw. In the lower jaw there are six in- 
cisors, two canines, and the same number of premolars and 
molars as in the upper jaw. 
As regards the digestive system of Mammals, the alimentary 
canal and digestive glands have on the whole the same general 
structure and arrangement as in man (p. 221, 222). Some very 
remarkable modifications, however, in the structure of the 
stomach and in the termination of the intestine occur in cer- 
tain 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 organs. 
The thorax contains chiefly the heart and lungs. The /eart is 
contained in a membranous sac—the pericardium, and con- 
sists of two auricles and two ventricles. The heart consists 
functionally of two sides, each having an auricle and a ven- 
tricle, 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 
