Circulatory System 



65 



SINUS VENOSUS 



Fig. 65. A. diagram of the primitive (fish) heart, as seen in a median longitudinal 

 section. Anterior is to the right. The course of blood in the heart (indicated by 

 arrows) takes the form of a letter S. (After Keith. Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



(see p. 67) which are seats of formation of leukocytes (Fig. 67). The 

 lymph-vessels in the wall of the intestine are especially concerned with 

 the absorption of the fatty constituents of the digested food. The mi- 

 nute globules of fat, carried by the watery lymph in a state of emulsion, 

 give it a milky white appearance. This fat -laden lymph is called chyle, 

 and the lymphatics which carry it away from the intestine are called 

 lacteals. 



Hemal "■glands" (""blood-glands") resemble lymph-nodules but 

 occur on the course of blood-vessels instead of on lymphatics, and 

 therefore are traversed by red blood, not by colorless lymph. The 

 spleen, a deep red organ of glandlike appearance but having no duct 

 connecting it with any other organ, is the largest of the hemal glands. 

 It is a seat of formation of red blood-cells (erythrocytes) and leuko- 

 cytes, and serves also to remove degenerating red blood-cells from the 

 circulation. It is usually attached to the digestive tube by a sheet of 

 membrane in the region of the stomach. 



Structure of Vessels. Arteries have relatively thick and strongly 

 muscular walls and, except at the junction of the main arteries with the 

 heart, they do not have internal valves. Walls of veins are less muscu- 

 lar and thinner than those of arteries, and a vein is of greater diameter 

 than the corresponding artery, the velocity of the blood-stream in the 

 vein being less than that in the artery. At intervals along the course of 

 a vein are valves, each consisting of flaps or pockets formed by the 



