CIRCULATORY SYSTEM 799 



end renal portal system. They bring blood from the tail and hind limbs 

 to the excretory organs (mesonephroi). The subcardinals of both sides 

 usually fuse in the middle line. The fusion is initiated by the appearance 

 of the interrenal veins, which now act as revehent vessels to carry blood 

 from the excretory organs to the postcava and to the anterior portion 

 of the postcardinals which have joined the anterior ends of the subcardi- 

 nals. In mammals there is also a change in the postcardinals and in the 

 renal portal system. 



In the lung fish Ceratodus there are some differences from the above 

 account. Here the cephalic portion of the right postcardinal loses its 

 connection with the vessels behind, a*nd acts as a vertebral vein, taking 

 the blood from the intercostal veins of that side back of the heart. The 

 caudal and the subcardinals form a continuous trunk, while the revehent 

 vessels form side branches. The caval portions of the postcardinals 

 grow back into the tail as paired vessels, forming no connection with 

 the caudal vein. In Protopterus there is no vertebral vein and the sub- 

 cardinals are not fused behind, while the advehent veins are connected 

 with the caudal. 



Pulmonary Veins: 



There may be various pairs of these. They carry blood from the 

 lungs to the left auricle of the heart. They never empty into the sinus 

 venosus. 



THE LYMPHATIC SYSTEM 



In addition to the arterial and venous divisions of the circulatory 

 system, all craniates develop lymph-vessels or lymphatics. 



These consist of a network of lymph capillaries which are inter- 

 woven with, but independent of, the blood-capillaries. The lymphatic 

 system is not closed like the blood-vascular system, for there are not 

 only definite lymph vessels, but there are large open spaces the lymph- 

 sinuses. Then, too, there are connections by little apertures, called 

 stomata, between the lymphatics and the coelom. 



Lymph sinuses are found beneath the skin, as in the frog, between 

 muscles, in the mesenteries, in the walls of the alimentary tract, around 

 the central nervous system, and in many other parts of the body. 



The lymph (which is practically the liquid part of the blood which 

 seeps through the blood vessel walls) is gathered in these sinuses and 

 then passes into more or less definite lymph vessels which, in turn, open 

 into the veins (Fig. 453). 



Leukocytes are added to the plasma from the various lymphatic 

 glands (Fig. 453), such as the tonsils, thymus, and spleen. 



In the lower craniates, such as the frog, lymph-hearts occur (Fig. 

 347). These are muscular dilations found in the course of certain 

 vessels. 



