Human Physiology. 203 



imbued with life ; but we see all round us, in the vegetative as 

 well as the animal kingdom, this process going forward. Life 

 lays hold of matter, transforms it, performs incomprehensible 

 feats of chemistry, and at length endows it with vitality. The 

 food taken into the stomach, in from one to five or six hours, 

 according to its digestibility, becomes living blood, and is sent 

 circulating with great rapidity through the system. 



There is in the body of an adult man about thirty pounds of 

 blood, one fifth of the weight of the whole body. Add the 

 weight of the heart, arteries and veins, and capillary system in 

 which it circulates, and you have at least one quarter of the 

 weight and bulk of the human body, filling every part of it. 



The heart, the great regulating organ of the circulation, is 

 placed in the centre of the body, embedded in and sur- 



Fig. 45. THE HEART. IDEAL 

 Fig. 44. HEART AND LUNGS. SECTION.* 



rounded by the lungs, because the first thing the living blood 

 requires to do is to breathe to be brought into contact 



* a, arch of aorta ; b, b, pulmonary arteries ; c, superior vena cava ^ 

 d, d, pulmonary veins ; e, right auricle ; f, tricuspid valves ; ,r, inferior 

 vena cava ; h, right ventricle ; /, left ventricle ; m, mitral valve ; n, left 

 auricle. 



