REPTILIA. 567 



more complex apparatus of Birds and Mammalia: the respiration 

 of Reptiles is consequently proportionately imperfect ; and hence 

 that coldness of their blood, and feebleness of muscular movement, 

 which are so characteristic of the entire class. 



The air required for purifying the blood is, of course, conti- 

 nually changed ; being alternately takeu into the lungs, and again 

 expelled in a deteriorated condition, by a mechanism which will be 

 found to vary in different Reptiles in accordance with the peculia- 

 rities of their organization. No Reptile possesses a diaphragm, 

 and, being destitute of this important muscle, the movements 

 whereby inspiration and expiration are accomplished are, in such 

 genera as are furnished with moveable ribs, entirely dependent upon 

 the mobility of the framework of the chest : the dilatations and con- 

 tractions of the thorax consequent upon the alternate elevation and 

 depression of the ribs being sufficient to ensure the inhalation and 

 expulsion of air, such is the case in the Serpent and the Lizard. 



In the AMPHIBIA, however, there are not even ribs developed, 

 or, if they exist at all, they are such mere rudiments as to be quite 

 useless as instruments of respiration ; and on the other hand, in 

 the CHELONIAN REPTILES, the large and expanded bones of the 

 thorax are so consolidated together, and so immoveably fixed to the 

 broad and osseous sternum, that respiration in the ordinary manner 

 F would be altogether impracticable. Under these circumstances, as 

 a compensation for the want of mobility in the chest, the os hyoides 

 and the muscles of the throat are converted into a kind of bellows, 

 by which the air is forced mechanically into the lungs, and they 

 are thus distended at pleasure. 



Any one who watches a Frog or a Tortoise with a little attention 

 will at once understand the mechanism by which this is effected. 

 The mouth is kept closely shut ; and the nostrils, which open 

 immediately into its cavity, are each provided with a muscular 

 valve so disposed as freely to permit the entrance of air into the 

 mouth, but also effectually preventing its return by the same chan- 

 nel. By this arrangement the descent of the hyoid apparatus fills 

 the mouth with air ; and the subsequent contraction of the broad 

 muscles of the throat, the nostrils and the pharynx being of course 

 both closed, forces the air into the opening of the larynx, and 

 distends the lungs, from which it ig again expelled by the pressure 

 of the abdominal muscles. 



The structure of the heart and the course of the circulation in 

 Reptiles afford interesting subjects for investigation. The heart 



