REPTILES, BIRDS, AND MAMMALS 



457 



mammalian characters. All but the most primitive mammals are vivip- 

 arous; the young are "born alive" in a more or less helpless state and are 

 cared for by the mother and fed on milk secreted by her mammary 

 glands. Mammals are warm-blooded like the birds and have a hairy coat 

 and sweat glands which help to maintain a constant body temperature. 

 Except for the birds, they are the most active and alert of vertebrates; 

 their metabolic rate is high, main- 

 tained by rapid respiration and an 

 efficient circulatory system with a 

 four-chambered heart. 



In structure mammals show 

 many advances over reptiles. The 

 brain is much larger, owing to the 

 great development of the cerebral 

 hemispheres; the remaining parts 

 are not much changed. The skull 

 has lost some of the bones present in 

 primitive reptiles and has been 

 modified from the type found in the 

 synapsid reptiles; the brain case is 

 much enlarged, it articulates with 

 the spine on two bony processes 

 (condyles) instead of one, the nost ri 1 s 

 open into an upper chamber cut off 

 from the mouth by a shelflike palate, 

 and the jaw is formed from a single 

 bone (the dentary) instead of from 

 several. The teeth are not all similar 

 as in reptiles but are differentiated 

 into four kinds. In primitive mam- 

 mals each tooth row contains 3 in- 

 cisors, 1 canine, 4 premolars, and 3 

 molars — 11 in all, or 44 in the entire 

 dentition. Most modern mammals 

 have lost a part of this complement. 

 A set of deciduous or "milk" teeth precedes the permanent ones. Reptiles 

 have one small bone (stapes) between the inner ear and the eardrum 

 (which lies at the surface) ; mammals have three. One of these is the old 

 reptilian bone, the other two (malleus and incus) are parts of the old 

 reptilian jaw hinge, which, with the eardrum, have sunk into the head 

 and been enclosed by the parts of the outer ear. An ear flap is developed 

 which concentrates sound waves. The legs of mammals are swung under 

 the body into a fore-and-aft position, elbows back and knees forward, so 



Fig. 28.22. The two sets of teeth of a gen- 

 eralized placental mammal. The first set 

 (black) develop in succession from front to 

 rear; before th& rearmost members of this 

 set (the permanent molars) have erupted, 

 a wave of replacement brings the second 

 set of teeth (white), again in anteropos- 

 terior succession. This wave, however, pro- 

 ceeds no further than the premolars; the 

 molars of the first set are not replaced. /, 

 incisors; C, canine; PM, premolars; M, 

 molars. /', C", PM', the teeth of the second 

 set. {Modified from Romer,' The Vertebrate 

 Body, by -permission W. B. Saunders 

 Company.) 



