MAMMALS. 101 



arrangement vary considerably in different mammals, and 

 the characters which they furnish are of great value in group- 

 ing the various forms. To express these characteristics 

 briefly a dental formula has been introduced, in which the 

 different kinds of teeth are indicated by initials, while the 

 number in either jaw is represented by a figure above or 

 below a horizontal line. Thus the permanent dentition of 

 man is expressed thus: i 8 , c |, pm f, m f ; which indi- 

 cates that in man there are two incisors, one canine, two 

 premolars, and three molars in each half of each jaw. The 

 pig has, i 3, c {, pm f , m f ; the cow, i J, c f, pm J, m f, 

 incisors and canines being absent from the upper jaw. 



The body-cavity is divided by a transverse muscular par- 

 tition, the diaphragm, into two chambers an anterior 

 pleural cavity containing the heart and lungs, and a poste- 

 rior peritoneal cavity in which is situated the stomach, 

 liver, intestine, etc. 



The heart, placed a little to the left of the median line, 

 is four-chambered, having, like that of the birds, two 

 auricles and two ventricles. Of these the auricle and ven- 

 tricle of the right side receive the blood from the body and 

 send it to the lungs, while those of the left side take the 

 blood as it comes from the lungs and send it through the 

 aorta to all parts of the body. The aorta, which bends 

 backward and to the left, represents the left arch of the 

 fourth pair of the primitive branchial vessels, the right of 

 the same pair being partially represented in the artery 

 (subclavian), which carries the blood to the right fore limb 

 a condition just the reverse of what occurs in the birds. 

 The fifth pair of arches form part of the arteries (pulmo- 

 naries) which convey blood from the heart to the lungs. 

 The blood of the mammals differs from that of all other 



