472 Comparative Morphology of Chordates 



is a median dermal bone, the episternutn or interclavicle, which 

 articulates with the right and left clavicles. 



Bones known as "abdominal ribs" or "gastralia*" occur in croco- 

 dilians, some lizards, and the tuatara. These are pairs of slender bones 

 lying superficially in the abdominal wall. They have no connection 

 with vertebrae and are apparently derived from the skin — "dermal 

 bones." They are therefore more in the nature of scales and not truly 

 endoskeletal. 



The structure of the skeleton of the reptilian trunk is highly sig- 

 nificant with reference to the mechanism of breathing. In amphib- 

 ians, with very short ribs or none at all and a body-wall which, beyond 

 the vertebral column, has no stiffening structures in it, air must be 

 forced into the lungs by muscular movements of the floor of the mouth 

 and the walls of the pharynx, the mouth and nostrils meanwhile being 

 closed. Air is thus pushed back into the trachea in much the same way 

 that food is pushed back into the esophagus. In reptiles the long ribs of 

 the anterior region of the trunk, several pairs of them anchored ven- 

 trally to the sternum, impart considerable rigidity to the body-wall. 

 The ribs, when in a resting position, slope somewhat sharply backward 

 (Fig. 363) from their points of attachment to the vertebrae. When 

 rotated forward by action of appropriate muscles, the diameter of the 

 coelomic space is increased, with the result that air pressure within the 

 lungs is decreased and external atmospheric pressure then forces air via 

 nostrils, mouth, and trachea back into the lungs — or, as we loosely say, 

 air is "sucked in." With return of the ribs to their resting position, air 

 is forced out of the lungs. (In turtles the enclosing of the body in the 

 "shell" necessitates another method of breathing. See p. 495.) 



It is significant, too, that the accentuation of ribs in the anterior 

 region of the trunk and their tendency to be shorter, or even entirely 

 lacking, on several vertebrae just anterior to the sacral region, result 

 in a condition closely approaching that in mammals, in which the 

 trunk is sharply divided into an anterior thorax with long ribs, most 

 of them meeting in a sternum, and a posterior abdominal region 

 (Figs. 451, 452). But in mammals the cavities of the two regions are 

 separated by a transverse partition, the diaphragm, which does not 

 appear in reptiles. The vertebral column of the mammalian trunk is 

 accordingly differentiated into an anterior rib-bearing thoracic region 

 and a posterior ribless (or nearly so) lumbar region, terms which may 

 be applied in some reptiles. 



The muscles of reptiles show much more differentiation than 

 those of anamniotes. In fishes there is very little local differentiation 

 of the transverse muscle-segments (myomeres) which extend in a uni- 



