CIRCULATION OF THE BLOOD. 235 



which are passed directly into the blood ; and lastly, 

 separation of useless matters as excrementitious. 



60'"). The chemical office of food is, to supply to 

 the body albumen and fibrin, the elements of blood ; 

 in order to counterbalance the waste continually going 

 on in the system, by the constant addition of all 

 those matters which enter into its composition ; and 

 secondly, to contribute to the formation of animal 

 matter, by the transformation of starch and other 

 substances of vegetable origin. 



606. Respiration has already been described (107) 

 as being, chemically speaking, a mere process of com- 

 bustion, in which carbon and hydrogen are burned at 

 the expense of the oxygen of the air ; this process 

 of combustion is carried on through the medium of 

 the blood, and goes on in all parts of the body. In 

 the lungs, the blood is exposed to the contact of a 

 quantity of atmospheric air ; oxygen is absorbed, 

 and carbonic acid given off; the blood thus charged 

 with free oxygen is sent, by the action of the heart, 

 to all parts of the body, in the innumerable minute 

 bloodvessels which terminate the arteries ; carbon and 

 a portion of hydrogen are taken up, and the oxygen, 

 which leaves the heart free, returns to it through 

 the veins, converted into carbonic acid and water ; the 

 former is at last given off from the blood, and expired 

 from the lungs, previous to the absorption of a new 

 quantity of fresh oxygen from the air. 



607. Two great objects are effected by the circula- 

 tion of the blood : the one is the removal of carbon 



