LYMPHATIC SYSTEM 327 



ing the excretory organs, there is developed a renal artery from the aorta for 

 each of the permanent kidneys. 



The anterior parts of both subcardinals separate from the portions formerly 

 connected with the excretory organs and are henceforth known as the azygos 

 vein on the right side, the hemiazygos on the left. Farther, either the azygos 

 or the hemiazygos may disappear, or, as in man, the hemiazygos may be inter- 

 rupted and the blood from the hinder part be returned to the heart by way of a 

 cross connexion (fig. 351) and the azygos of the right side. In such a case the 

 anterior part of the hemiazygos is called the superior intercostal vein. The 

 abdominal veins are important in fcetal life, bringing blood from the placenta 

 to the embryo. 



In the lower vertebrates (monotremes, marsupials, ungulates, insectivores 

 and bats) the Cuvierian ducts of both sides persist as in the non-mammalian 

 classes. In whales, edentates, carnivores and primates an innominate vein 

 extends across from the carotid-subclavian trunk of one side to the other (fig. 

 351, jB) so that all of the blood is returned to the heart by means of the base of 

 the right trunk, now known as the precava (vena cava anterior). All that 

 persists of the left Cuvierian duct is the coronary vein, returning blood from 

 the wall of the heart to the right atrium. 



THE LYMPHATIC SYSTEM 



The lymphatic system consists of (i) a series of lymph- vessels 

 which penetrate all parts of the body; (2) of pulsating portions of 

 these vessels, the lymph hearts ; and (3) peculiar aggregates of con- 

 nective tissue, leucocytes and lymph-vessels which are grouped under 

 the general head of lymph glands. 



There are different views as to the morphology of the blood and lymph sys- 

 tems. According to one (Marcus) the lymph-vessels were primitively connected 

 with the ccelom and have only secondarily come into relations with the blood- 

 vascular system. Others think that both blood- and lymph-vessels have arisen 

 from extracoelomic spaces, from which, by modification and specialization, the 

 two systems have been differentiated. The fact that in many invertebrate- 

 there is but a single system, best compared with the lymph system of the vertes 

 brates, and that, even in the Crustacea, lymphatic and blood systems are but 

 partially differentiated, is of interest in this connexion. 



The lymph- vessels are, in part, capillary in character, with walls 

 of endothelium. The larger ducts and still larger sinuses are 

 strengthened by the same layers of smooth muscle fibres and elastic 

 and fibrous tissue as occur in the blood-vessels (p. 288). The capil- 

 laries have numerous anastomoses, but the larger vessels are said to 

 terminate blindly, while, at least in the higher vertebrates, some may 

 connect with the ccelom by minute openings (stomata) in the peri- 

 toneal wall. In the mammals and, to a less extent in the birds, the 



