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 vercebrates. 
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 inferior 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 io 
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 from one part of the body to another. 
CIRCULATORY System. Lhe 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 
