216 



THE CIRCULATORY SYSTEM 



[CH. XIX. 



The blood is oxygenated in the lungs, and the formation of oxy- 

 hsemoglobin 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 oxy- 

 haemoglobin ; 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 

 in the thorax three large branches for the supply of the head, neck, 

 and upper extremities; it then traverses the thorax and abdomen, 

 giving off branches, some large and some small, for the supply of the 



FIG. 196. Transverse section through a 

 large branch of the inferior mesenteric 

 artery of a pig. e, Endothelial mem- 

 brane ; i, tunica elastica interna, no 

 subendothelial layer is seen; m, mus- 

 cular tunica media, containing only a 

 few wavy elastic fibres; e, e, tunica 

 elastica externa, dividing the media 

 from the connective-tissue adventitia, 

 a. (Klein and Noble Smith.) x 350. 



FIG. 197. Minute artery 

 viewed in longitudinal 

 section, e, Nucleated 

 endothelial membrane, 

 Avith 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. 



various organs and tissues it passes on its way. In the abdomen it 

 divides into two chief branches, for the supply of the lower ex- 

 tremities. The arterial branches wherever given off divide and sub- 

 divide, until the calibre of each subdivision becomes very minute, and 



