784 



COMPARATIVE ANATOMY 



with each other, but just before reaching the tail a large branch is given 

 off, in fact, the branch is much larger than the aorta itself from which it 

 arises. This large branch is called the vitelline artery. It is the vitelline 

 arteries that carry the blood back to the vascular area from which it was 

 brought by the vitelline veins. 



The heart we have just been describing is that of the chick. In 

 cyclostomes and fishes (except the dipnoi) there is what is known as a 

 branchial or venous heart (Fig. 446). All of the blood which enters such 

 a heart is venous blood. This venous blood is pumped directly to the 

 gills where it loses its carbon dioxide and takes up oxygen before being 

 distributed to the various parts of the body. The important thing to 

 note is that in such cases the blood only passes through the heart once 

 in making its complete circuit. It is not, however, correct to consider 

 the embryo of higher forms as being the same as this type of "one-heart- 

 circulation," for only oxygenated blood passes through the heart in such 

 embryos when it is in this stage. 



In the dipnoi and amphibia (Fig. 446), where lungs are formed 

 which take up part of the work of the gills, the heart divides in an 

 arterial or systemic and a venous or respiratory half. This division is 

 caused by a septum or partition in the auricle which divides the cham- 

 bers. It will be remembered that blood always enters through a vein 

 and always enters into a sinus or an auricle of the heart. The venae 

 cavae which return the systemic blood to the heart, therefore, empty 



srpt.ao.pulm. 



ao.d. 



anasl 

 cod 



H. 



Fig. 447. 



