368 SYSTEMATIC ZOOLOGY. 



man is expressed thus: if, i, pwf, raf ; 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, if, cf, praf, raf ; the cow, i, cf, pwf, mj, 

 incisors and canines being absent from the upper jaw. 



The body-cavity is divided by a transverse muscular 

 partition, the diaphragm (p. 309), into two chambers an 

 anterior pleural cavity containing the heart * and lungs, 

 and a posterior peritoneal cavity in which are 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 sixth 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 

 fo ms in that the red corpuscles (p. 314) are usually cir- 

 cular in outline and are not nucleated. 



The monotremes form the only exceptions to the state- 

 ment that the mammals bring forth living young. They 

 lay eggs, one species having the eggs about the size of a 

 pigeon; but the young which are hatched from these eggs 



* The heart, inside the pericardium, is not actually inside the 

 pleural cavity, which really contains the lungs alone. 



