572 Comparative Morphology of Gtiordates 



General Form 



Typical terrestrial mammals differ considerably from quadruped 

 reptiles (leaving Chelonia out of account), as to their general form. 

 The trunk in mammals is relatively shorter, the neck longer and more 

 slender, and the tail is so much reduced in diameter that it appears 

 more as a slender appendage of the trunk instead of a postsacral con- 

 tinuation of it as in most amphibians and reptiles. The legs are rela- 

 tively longer and are so placed (see p. 566) that the elbow-joint points 

 backward and the knee-joint points forward. The most striking internal 

 difference is the division of the mammalian coelomic space into two 

 major cavities, the abdominal (posterior) and the thoracic (an- 

 terior), by a transverse muscular (therefore contractile) partition, the 

 diaphragm (Fig. 17). When relaxed, the diaphragm is convex 

 anteriorly. Its rhythmic contractions are an important part of the 

 mechanism of breathing. The thoracic space includes the median 

 pericardial cavity and a pair of pleural cavities containing the 

 right and left divisions of the lungs. 



