VER TEBRA TES. 313 



parts, and becomes loaded anew with carbon dioxide and 

 other waste, changing color again to the dark red. From 

 this account it will be seen 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 verte- 

 brates may be derived, simply by enlarging some vessels 

 and suppressing others. Some of the changes involved 

 may be made out from the diagrams (fig. 117) in compari- 

 son 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, pulmonary arteries going to these 

 organs are developed from the hinder pair of branchial 

 arteries (fig. 117, B-E, P). 



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 with the frog. By means of a peculiar valve 

 the red blood is sent to the body, the dark blood to the 

 lungs. 



