696 EEPTILIA. 



effectually preventing its return by the same channel. By this arrange- 

 ment the descent of the hyoid apparatus fills the mouth with air ; and 

 the subsequent contraction of the broad muscles of the throat, the nos- 

 trils and the pharynx being of course both closed, forces the air into the 

 opening of the larynx, and distends the lungs, from which it is again 

 expelled by the pressure of the abdominal muscles. 



(1951.) The structure of the heart and the course of the circulation 

 in Reptiles afford interesting subjects for investigation. The heart 

 consists of three cavities, namely, a strong and muscular ventricle 

 (fig. 342, a), and two membranous and very capacious auricles, both of 



Fig. 342. 



which communicate by valvular openings with the ventricular cavity. 

 The right auricle (6) receives the venous blood from all parts of the body 

 through the venae cavae (n, o, p), the terminations of which are guarded 

 by strong valves ; the left auricle (c) is appropriated exclusively to the 

 lungs, from which it receives arterial blood through the pulmonary 

 veins (m m). It is obvious, therefore, that the ventricle receives two 

 kinds of blood from the two auricles venous blood from the systemic 

 auricle, and arterial blood from the pulmonic auricle ; and as the interior 



