A COMPARATIVE STUDY OF THE CIRCULATORY SYSTEM 259 



artery and the aorta. The former conveys the blood from the right 

 ventricle to the lungs, and the latter from 

 the left ventricle to all parts of the body. 



If the multitude of blood-vessels con- 

 stituting the different divisions of the cir- 

 culatory system are taken and moulded 

 into single channels, a system of tubes is 

 formed such as is represented in the ad- 

 joining schema (Fig. 128). In studying 

 this diagram more closely, we find that 

 a droplet of blood leaving the left ven- 

 tricle first enters the central arterial 

 trunk, or aorta, whence it escapes into 

 either the blood-vessels of the head or 

 those of the trunk and lower extremities. 

 In either case, it must first traverse the 

 arteries, then the arterioles and finally, 

 the capillaries. Having attained the 

 other side of these extremely fine tubules, 

 it enters the venules and then the veins 

 to be eventually conveyed into the right 

 auricle. The venous trunks in the vi- 

 cinity of the heart are designated as the 

 superior and inferior cava respectively. 



This extensive system of blood-vessels 

 which supplies all the tissues of the body 

 with the exception of the lungs, consti- 

 tutes the greater, or systemic circuit. It 

 embraces two specialized smaller divi- 

 sions, namely, the coronary and portal 

 circuits. The former arises from the 

 root of the aorta as the coronary artery 

 and ends in the right auricle as the 

 coronary vein or sinus. The coronary 

 blood-vessels have to do solely with the 

 nutrition of the heart. The portal cir- 

 cuit begins with the arteries supplying 

 the so-called portal organs, namely, the 

 spleen, pancreas, liver, stomach, and in- 

 testine. Having traversed the different 

 capillary networks of these organs, the 

 blood is collected by a single channel, 

 known as the portal vein, and is then 

 conducted r to the liver, whence the he- 

 patic veins convey it into the inferior 

 vena cava. The portal circuit, therefore, is concerned chiefly with the 

 processes of digestion and absorption. 



Fio. 128. SCHEMA OF THE 

 CIRCULATION. 



A, aorta; Ar, arteries; Art, 

 arterioles; AC, arterial capillaries; 

 C, capillaries; VC, venous capil- 

 laries; Ven, venules, Ve, veins; 

 VCS, vena cava superior; VCJ, 

 vena cava inferior; RA, right 

 auricle; RV, right ventricle; LA, 

 left auricle; LV, left ventricle; 1, 

 tricuspid valve; 2, mitral valve; 

 3, pulmonary semil. valve; 4, aortic 

 semil. valve; PA, pulmonary 

 artery; L, lungs; PV, pulmonary 

 veins; PO, portal organs; PV, por- 

 tal vein; HA, hepatic artery; Li, 

 liver; HV, hepatic vein. 



