214 



THE CIRCULATORY SYSTEM 



[CII. XVIII. 



right auricle by the veins, the arteries and veins being continuous 

 with each other at the far end by means of the capillaries. 



From the right auricle the blood passes to the right ventricle, 

 then by the pulmonary artery, which divides into two, one for each 

 lung, then through the pulmonary capillaries, and through the 

 pulmonary veins (two from each lung) to the left auricle. From 

 here it passes into the left ventricle, which brings us back to where 

 we started from. 



The complete circulation is thus made up of two circuits, the one, 

 a shorter circuit from the right side of the heart to the lungs and 

 back again to the left side of the heart ; the other 

 and larger circuit, from the left side of the heart 

 to all parts of the body and back again to the 

 right side. The circulations through the lungs 

 and through the system generally are respectively 

 named the Pulmonary and Systemic or lesser 

 and greater circulations. It will be noticed also 

 in the same figure that a portion of the stream 

 of blood having been diverted once into the 

 capillaries of the intestinal canal, and some other 

 abdominal organs, and gathered up again into a 

 single stream, is a second time divided in its 

 passage through the liver, before it finally reaches 

 the heart and completes a revolution. This sub- 

 ordinate stream through the liver is called the 

 Portal circulation. A somewhat similar accessory 

 circulation is that through the kidneys, called the 

 Renal circulation. The difference of colours in 

 fig. 222 indicates roughly the difference between 

 arterial and vencws blood. The blood is oxygen- 

 ated in the lungs, and the formation of oxy- 

 haemoglobin gives to the blood a bright red colour. 

 This oxygenated or arterial blood (contained in 

 the pulmonary veins, the left side of the heart, and systemic arteries) 

 is in part reduced in the tissues, and the deoxygenated haemoglobin 

 is darker in tint than the oxyhsemoglobin ; this venous blood passes 

 by the systemic veins to the right side of the heart and pulmonary 

 artery to the lungs, where it once more receives a fresh supply of 

 oxygen. 



N.B. It should, however, be noted that the lungs, like the rest of the body, 

 are also supplied with arterial blood, which reaches them by the bronchial arteries. 



The Arteries. 



The arterial system begins at the left ventricle in a single large 

 trunk, the aorta, which almost immediately after its origin gives off 



FIG. 223. Minute artery 

 viewed in longitudinal 

 section, e. Nucleated 

 endothelial membrane, 

 with faint nuclei in 

 lumen, looked at from 

 above, i. Elastic mem- 

 brane, m. Muscular 

 coat or tunica media, 

 a. Tunica adventitia. 

 (Klein and Noble 

 Smith.) x 250. 



