CIRCULATION. 



639 



blood which has served the purposes of nu- 

 trition, and the other uses for which the blood 

 is destined throughout the body, on being re- 

 turned to the heart, is directed by the cavities 

 on the right side of that organ to the lungs, and 

 made to pass through them before returning 

 to the left side of the heart to repeat its course 

 through the nutritive vessels of the body. 



In all those animals in which there exists a 

 disposition of the heart and bloodvessels such 

 as that described, the circulation is said to be 

 double, because the blood is moved in two 

 circles at once, and the respiration is said to 

 be complete, because the whole of that blood 

 which has passed through the nutritive vessels 

 of the body is subjected to the respiratory 

 action of air in the lungs. 



The blood returned from the lungs of a 

 bright red colour, or arterial blood, on being 

 expelled from the left ventricle (Jig. 312, H) 



Fig. 312* 



Circulation in Man. 



* In all the figures relating to the circulation in 

 different animals the same letters indicate corres- 

 ponding parts as follows : 



Iff the heart or the common ventricle j h, the 

 common auricle ; 



A, the aorta or trunk of the systemic arteries ; 



a, its branches ; a*, the carotids. 



V f the great systemic veins or vena cava infe- 

 rior j v, its branches ; *, the vena cava 

 superior ; c, the capillary vessels ; 



P, the pulmonary artery ; p, the pulmonary 

 vein ; 



B, the branchial artery ; b, the branchial vein ; 

 D, the ductus arteriosus ; d, ductus venosus ; 



f, foramen ovale ; 

 U, umbilical arteries ; , umbilical vein j 



by the muscular contraction of that cavity, 

 passes into the aorta or great artery of the 

 system (A), and is distributed in various pro- 

 portions to all parts of the body by the 

 branches of the aortic trunk (a) and their in- 

 finitely minute ramifications. The smallest 

 arteries lead, by an intermediate set of minute 

 tubes to which the name of capillary vessels is 

 given, into the systemic veins (u), all of which 

 (the veins of the intestinal canal excepted) join- 

 ing gradually together into larger and fewer 

 branches, form at last the great trunks of the 

 superior and inferior venae cavae ( F, v*), which 

 carry back to the centre of the circulation the 

 whole of the blood that had passed from the 

 left ventricle into the aorta. 



In passing from the arteries to the veins - 

 through the capillary vessels, the properties of 

 the arterial blood are changed ; its colour is 

 altered from bright scarlet to dark purple, 

 it expends some of its substance in the nou- 

 rishment of the textures, and a considerable 

 quantity of its thinner part transudes through 

 the small vessels, constituting the lymph that 

 is taken up by the absorbent vessels. The 

 venous or dark blood, as it approaches the 

 heart upon its return, has its composition fur- 

 ther changed by its admixture with the chyle 

 or imperfectly formed blood, which is the pro- 

 duct of digestion, and which is poured along 

 with the lymph from the thoracic duct into 

 the great veins of the head and superior ex- 

 tremities. 



By the changes thus produced in its com- 

 position, &c., the venous blood which returns 

 to the heart is rendered unfit for nutrition, 

 until it has been acted upon by the atmos- 

 pheric air in the lungs, which restores to it its 

 bright red colour and arterial composition and 

 properties. 



The great systemic veins are therefore con- 

 nected with the right side of the heart (If), 

 and the stream of venous blood brought by 

 them to the right auricle (h'\ next issues from 

 the heart by the pulmonary artery (P), into 

 which it is propelled by the contraction of the 

 right ventricle (H') as it passes through that 

 cavity. The minute branches of the pulmo- 

 nary arteries and veins ( P, p), and the capil- 

 lary vessels by which they communicate with 

 one another, are wholly distributed on the 

 membrane lining the air-cells of the lungs. 

 In passing through these vessels then, the 

 venous blood is exposed to the action of the at- 

 mospheric air contained in the pulmonary cells ; 

 and, after having acquired arterial properties, 

 is returned to the centre of the circulation by 



/, arteries of the intestine or alimentary canal ; 



, the coeliac artery ; 

 L, vena port<fi j I, hepatic vein ; I*, hepatic 



artery ; 

 K, advehent renal veins ; k, renal veins ; k*, 



renal artery. 



In those instances in which the parts are double, 

 those on the right side are distinguished by the 

 accentuation of the letters indicating them, thus 

 P' right pulmonary artery, P left ditto. 



We beg to remind the reader that most of these 

 figures are merely plans, and that strict anatomical 

 accuracy is not to be looked for in them. 



