BLOOD SYSTEM. 



36 



of a diaphragm in mammals. The aeration of the bloo 

 of reptiles, however, is very imperfect, for reptiles ar 

 cold-blooded. This, however, is due not so much t 

 imperfect respiration as to the coarseness of the spong 

 structure of the lungs, and therefore the smaller surfac 

 of contact of blood with air. It is due, also — as we sha 

 see hereafter — to the fact that only a part of the bloo 

 is exposed to the air. 



Tortoise. — There is one order of reptile the me 

 chanics of whose respiration is entirely peculiar — viz., th 

 tortoise. Like other reptiles, they have no diaphragrr 

 and therefore can not have this kind of breathing, bu 

 neither can they have costal breathing, for their ribs ar 

 immovably consolidated with the shell. The problen 

 then, is how to expand the body cavity. As might b 

 expected, therefore, the breathing of tortoises and turtle 

 is exceptional. It is effected partly by muscular sheet 

 which arise from the shell and pass over the viscera 

 including the lungs, and by contraction compress ther 

 and force out the air; and partly (especially in Ian 

 tortoises) by movements of the shoulder and hip girdles 

 In most vertebrates the shoulder girdle is movable, bu 

 the hip girdle is fixed; but in tortoises both girdles ar 

 movable. In inspiration the shoulder girdle is draw; 

 forward and the hip girdle backward, and the bod 

 cavity is thus enlarged. In expiration there are con 

 trary movements — i. e., backward of the shoulder girdl 

 and forward of the hip girdle, and the body cavity i 

 contracted.* 



Amphibians. — The respiration of these, e. g., th 

 frog, is far inferior to even that of reptiles. (1) Th 

 lung is no sponge at all, but only a sac. There is n< 

 trachea and very short bronchi. The glottis opens fror 



* Charbonnel-Salle, An. des, Sci. Nat., vol. xv, art. 6, li 

 25 



