266 



THE CIRCULATION. 



Fig. 77. 



these animals the respiratory process is not a very active one ; but 

 the gills, which are of small size, being the only respiratory organs, 

 all the blood requires to pass through them for purposes of aeration. 

 The heart here is a single organ, destined only to drive the blood 

 from the termination of the venous system to the capillaries of the 

 gills. 



In reptiles, the heart is composed of two auricles, placed side by 

 side, and one ventricle. (Fig. 77.) The venas cavse discharge their 



blood into the right auricle (a), 

 whence it passes into the ventricle 

 (t-). From the ventricle, a part of it 

 is carried into the aorta and distri- 

 buted throughout the body, while a 

 part is sent to the lungs through the 

 pulmonary artery. The arterialized 

 blood, returning from the lungs by 

 the pulmonary vein, is discharged 

 into the left auricle (b), and thence 

 into the ventricle (c), where it 

 mingles with the venous blood 

 which has just arrived by the vense 

 cava3. In the reptile, therefore, the 

 ventricle is a common organ of pro- 

 pulsion, both for the lungs and for 

 the general circulation. In these 

 animals the aeration of the blood in 

 the lungs is only partial ; a certain 

 portion of the blood which leaves 

 the heart being carried to these organs, just as in the human subject 

 it is only a portion of the blood which is carried to the kidney by 

 the renal artery. This arrangement is sufficient for the reptiles, 

 because in many of them, such as serpents and turtles, the lungs 

 are much more extensive and efficient, as respiratory organs, than 

 the gills of fish ; while in others, such as frogs and water-lizards, 

 the integument itself, which is moist, smooth, and naked, takes an 

 important share in the aeration of the blood. 



In quadrupeds and the human species, however, the respi- 

 ratory process is not only exceedingly active, but the lungs 

 are, at the same time, the only organs in which the aeration of 

 the blood can be fully accomplished. In them, accordingly, we 

 find the two circulations, general and pulmonary, entirely dis- 



ClRCULATION OF REPTILES. a. 



Right auricle. 6. Left auricle, c. Ventricle. 

 d. Lungs, e. Aorta. /. Veua cava. 



