BLOOD SYSTEM. ,g 9 



the fish has the advantage of aerating the whole of the 

 blood, and therefore sending only aerated blood to the 

 tissues, while in reptiles only a part of the blood is 

 aerated, and therefore mixed blood goes to the tissues. 

 Per contra, however, the fish has again the disadvan- 

 tage of the blood not coming back from the gills to the 

 heart to receive fresh impulse, so that the heart has to 

 do the double duty of sending the blood by one impulse 



Fig. 268.- 



-A, heart and gill arches of a fish ; B, one arch with fringe (after 

 Owen) ; H, the heart. Dark blood is shaded. 



through two capillary circulations. In a morphological 

 point of view the reptile is undoubtedly the higher form 

 of circulation, for it is a transition to the still higher 

 forms. The three-chambered heart is a transition from 

 the two-chambered heart of the fish to the four-cham- 

 bered heart of birds and mammals, and what might be 

 called the one-and-one-half circulation of reptiles a tran- 

 sition from the single circulation of the fish to the double 

 circulation of the bird and mammal. 



