DIGESTION.] PHYSIOLOGY. 103 



each breath helps to purify the blood, and the pump- 

 ing of air in and out of the chest changes the impure, 

 hurtful, venous, to pure, refreshing, arterial blood ; tlje 

 blood breathes air in the lungs, that all the body may 

 may in turn breathe blood. 



HOW THE BLOOD IS CHANGED BY FOOD : 



DIGESTION. §VII. 



45. The blood is not only purified by air, it is also 

 renewed and made good by food. The food we eat 

 becomes blood. But our food, though frequently moist, 

 is for the most part solid. We cut it into small pieces 

 on the plate, and with our teeth we crush and tear it into 

 still smaller morsels in our mouth. Still, however well 

 chewed, a great deal of it, most of it in fact, is swal- 

 lowed solid. In order to become blood it must first be 

 dissolved. It is dissolved in the alimentary canal, 

 and we call the dissolving digestion. Let us see how 

 digestion is carried on. 



Your skin, though sometimes quite moist with per- 

 spiration, is as frequently quite dry. The inside of 

 your mouth is always moist — very frequently quite 

 filled with fluid ; and even when you speak of it as 

 being dry, it is still very moist. Why is this ? The 

 inside of your mouth is also very much redder than 

 your skin. The redness and the moisture go together. 



In speaking of the capillaries, I said that almost 

 all parts of the body were completely riddled with 

 them, but not quite all. A certain part of the skin, 

 for instance, has no capillaries or blood-vessels at all. 

 You know that where your skin is thick, you can shave 

 off pieces of skin without " fetching blood ; " if your 





