148 ELEMENTS OF COMPARATIVE ZOOLOGY. 



that in the fish only blood charged with impurities passes 

 through the heart. 



From the arrangement of blood-vessels found in the fishes 

 (sharks) all the conditions found in the higher vertebrates 

 may be derived, simply by enlarging some vessels and sup- 

 pressing others. Some of the changes involved may be 

 made out from the accompanying diagrams (Fig. 69) in 

 comparison with your dissections, the explanatory statement 

 being made that in embryo birds and mammals paired 

 branchial arteries occur, while in the adult this symmetry 

 is largely lost. 



One point particularly to be mentioned is that with the 

 development of lungs, arteries going to these organs are 

 developed from the hinder pair of branchial arteries. 



When the gills are lost and the lungs function as respi- 

 ratory organs, the conditions of the circulation are changed. 

 The blood, in leaving the heart, passes partly to the various 

 parts of the body, partly to the lungs. That going to the 

 latter organ loses its carbon dioxide, and takes up oxygen 

 and changes to bright red. It now returns along with 

 blood from other parts to the heart, which therefore now 

 receives both light and dark blood and forces the same out 

 again. But when the lungs are developed the auricle of 

 the heart divides, and one auricle receives the dark, the 

 other the light blood, both emptying their contents in turn 

 (in frogs and reptiles) into the single ventricle. It was 

 therefore formerly thought that the blood sent out through 

 the ventral aorta must necessarily be mixed; but this is 

 not the case. By means of a peculiar valve the red blood 

 is sent to the body, the dark blood to the lungs. 



As has already been mentioned, in crocodiles, birds, and 

 mammals the ventricle is also divided, and now one half of 



