88 Anatomy of the Rabbit. 



A similar division of the circulatory organs occurs as a homoplastic 

 modification in birds, which, it will be observed, are also warm- 

 blooded vertebrates. 



In general, the blood which is distributed to the various parts 

 of the body passes through but one set of capillary vessels, and is. 

 then returned through the systemic veins to the heart. In all 

 vertebrates, however, a special portion of the systemic venous 

 circulation is set aside as the hepatic portal system (Fig. 42), 

 distinguished by the possession of a second series of capillary 

 vessels ramifying in the liver. Thus, in the rabbit, the blood ^ 

 distributed to the stomach, spleen, and intestine through the coeliac, 

 superior and iaferior mesenteric arteries, is collected into a main 

 intestinal vessel, the portal vein, and the latter, approaching the 

 liver through the lesser omentum, divides in that organ into a series 

 of portal capillaries. The portal capillaries, like the systemic 

 capillaries proceeding from the hepatic artery, unite in the tribu- 

 taries of the hepatic veins. In lower vertebrates, although not it> 

 the mammalia, a second system of venous capillaries occurs in 

 connection with the kidneys and is known as the renal portal 

 system. 



The ultimate function of the vascular system is connected with 

 interchange of materials in the tissues. This is brought about 

 through the medium of microscopic capillaries, the gross parts 

 of the system being concerned with transportation of materials 



FUNCTIONS OF ^^"^ ''"^ P^"^* "^ '^^^ '^"^^ ^° another. 



CIRCULATORY SYSTEM. The propulsive action of the heart is 



muscular and rhythmic, contraction, or 

 systolic phases, alternating with expansion, or diastolic phases 

 (cf. p. 63). The flow is maintained in one direction principally by 

 atrioventricular valves of the heart, and by the semilunar valves 

 the of the aorta and pulmonary arteries, though there are also valves 

 in the course of some of the veins. The arteries are tubes with 

 .thickened elastic walls. They are expanded by the impulse of 

 blood from the heart, contraction of which is followed by a pulse 

 wave in the arteries. The passage of blood into the capillaries 

 takes place more slowly and uniformly, while the arteries contract 

 to their previous diameter. In the return, of the blood the veins 

 are largely passive, acting merely as closed channels connecting the 



