1st; CHORD ATA. 



and pulmonary artery to the lungs. Becoming arterial there, it flows by the 

 pulmonary vein into the left auricle and ventricle, and thence by the aorta into 

 the systemic circulation. For details see fig. 538. 



llVsides blood-vessels, lymph vessels occur as complements of the venous 

 svsti-m. The fluids which collect in the spaces of the connective tissue are taken 

 by them and carried (thoracic duct) into the large venous trunks. Usually the 

 action of the heart and the movements of the body are sufficient to cause the 

 flow of this lymph, but special lymph hearts may occur. The lymph vessels 

 distributed to the digestive tract play an important role, since they serve in the 

 resorption of digested food. They are called chyle ducts because their contents, 

 the chyle, rendered white by oil globules at the time of digestion, distinguishes 

 them from other lymphatics. The most important features of lymph and blood 

 have already been noticed (p. 78). In special places small bodies, the lymph 

 glands, are inserted in the course of the lymph vessels, in which lymph corpuscles 

 arise. Among these from its structure is to be enumerated the spleen, colored 

 bright red by its rich blood supply. 



The sexual and excretory organs are so closely associated that they are 

 generally united as the uro genital system. The sexual products are formed 

 in the embryo in a special region of the peritoneal epithelium on either 

 side of the vertebral column. These primordial cells arise from the ento- 

 derm, migrate into the epithelium and then early leave this position, and 

 sink into the underlying connective tissue (fig. 34), forming in the male 

 glandular tubes, in the female cords which break up into numbers of 

 round follicles, each containing a single larger cell, the ovum. In the 

 male the gonads thus formed are compact and frequently oval testes; in 

 the female they are looser and follicular ovaries. 



In many fishes the sexual cells pass out by way of the body cavity and 

 the abdominal pores, and in this case a part of the ccelom may be cut off as 

 a special vas defer ens or oviduct. In mo.-t vertebrates the ducts are formed 

 from a part of the nephriclial system. The urogenital system thus formed 

 recalls that of the annelids and in both there is the same origin of gonads 

 and nephridia from the ccelomic epithelium. As the lower vertebrates 

 (Amphibia and Elasmobranchs) show, the first stage of the excretory 

 system is furnished by segmentally arranged canals (ncphridial tubules} 

 which open into the body cavity by ciliated funnels, the nephrostomes. 

 In many forms the nephrostomes persist throughout life, in others they 

 degenerate. In the higher vertebrates they are usually not formed, a 

 fact connected with the loss of the excretory function of the coelom and 

 its assumption by the blood system. This has entered into connexion 

 in a characteristic way with the nephridial tubules, and especially by the 

 formation of ^lomcmli, networks of blood capillaries which, pushing the 

 wall of the tubules before them, produce the Malpighian bodies. Rarely 

 there is a rich blood vascular development in the wall of the body cavity, 



