OF THE BLOOD ATTD CIRCULATION. 203 



mixture soon takes place by means of a special artery which 

 passes from the pulmonary artery to the aorta. [The reptiles 

 have a heart with one ventricle, and two auricles ; the right 

 auricle receives the impure venous blood from the body, the 

 left auricle receives the pure arterial blood from the lungs, and 

 both pour their contents into the same ventricle, where they 

 are minied together. This mixed blood is transmitted by the 

 ventricular contractions partly into the lungs and partly into 

 the body ; in the crocodile a partial partition divides the ven- 

 tricle into a right side and a left side, as in birds and mammals. 

 Fig. 219 is a plan of the circulation in reptiles; the arrows 

 indicating the course of the blood. 



Lesser circulation. 



Yena cava. 



Heart. 



Aorta. 



' Single ventricle. 



Greater circulation. 

 Fig. 219. Circulation in reptiles. 



[ 367. In fishes the heart possesses two cavities, an auricle 

 and a ventricle, and only receives and transmits venous blood ; 

 it therefore represents the right side of the heart of birds and 

 mammals. The venous blood returned by the systemic veins 

 is poured into the auricle and ventricle, from whence a highly 

 elastic artery arises, which divides into five pairs of branches ; 

 these branchial arteries distribute the blood throughout the 

 gills ; from these organs it is conveyed into a large single vessel, 



