70 THE HUMAN FRAME. 



heart with less impetus, there is always twice as much blood in the veins 

 as in the arteries. As the blood in this discolored state has lost some of 

 its vital power, it is drawn through the lungs, and its color restored; 

 but on its passage back to the heart, it also receives a supply of a new 

 fluid, extracted from the food in the stomach and intestines: the process 

 of digestion is not within our province to describe. 



The bones are provided as a substantial frame for the body, giving firm- 

 ness and shape, and acting as levers for themuscles, made ofproportionate 

 strength, as weight or other circumstances require. The bones in the 

 legs of all animals are solid, and men's bodies being supported by two 

 limbs only, the bones of those limbs are therefore more solid than those 

 of quadrupeds. There are 218 separate bones in the human body, and 

 they classed under those of the head, the trunk, and the extremities. 



THE HUMAM LUNGS. 



The human lungs, like those of the inferior animals, vulgarly called 

 "the lights," are soft spongy substances, which, when healthy, will float 

 in the water (this test, as a proof of life having once existed in the foetus, 

 is very doubtful). Their use is to assist in the purification of the blood. 

 In fishes this duty is performed by the gills : and in insects, no air being 

 admitted by the mouth, their blood is ventilated through the medium of 

 small holes arranged along their sides. When " holding our breath," 

 we soon experience a feeling of suffocation ; this is merely a nervous 

 impression, produced by the blood passing impure through the lungs 

 to the left side of the heart ; and it indicates the necessity of respiring 

 fresh air to purify that fluid. The sensible change which the blood 

 undergoes in passing through our lungs, is observed in its colour, 

 characterised as veinous blood. It enters the lungs of a blackish or 

 deep purple colour, but in leaving them it is of a bright vermillion red, 

 and it is then called arterial blood. This change is owing to the action 

 of the inhaled air. The lungs, with the exception of the air tubes 

 (branches of the windpipe that perforate them in every direction), are 

 one mass or net work of blood vessels. These, when approaching to the 

 surface of the lungs, divide into an infinitude of small branches, the 

 coats of which are so extremely thin, that the air we breathe readily acts 

 through them, and makes the requisite changes. The circulation of the 

 blood, from the time it leaves the lungs until it returns again, is very 



