XIII. THE ANIMAL 269 



The increase in the volume of carbon dioxide during respiration is, 

 however, almost always less than the decrease in that of oxygen. The 



ratio, '. 2 ev Ve , is known as the respiratory quotient. It varies with 

 O 2 absorbed 



circumstances, particularly with the food and the amount of muscular 

 exertion. It is usually about 0*9. The disappearance of oxygen is prob- 

 ably due to its being used to oxidise hydrogen to water (say from fats 

 and proteids in the food), and also in the formation of certain waste 

 products, e.g., urea. 



Seat of oxidation in the body. Although almost all the oxygen ab- 

 sorption of the body occurs in the lungs (a small amount also through 

 the skin), the act of union with the combustible matter derived from 

 the food and the consequent heat production do not occur there. It 

 is not in the lungs nor even in the blood that the combustion and for- 

 mation of carbon dioxide occur, but in the tissues themselves. 



Bones. Bone consists mainly of an earthy substance permeated 

 by an albuminoid known as ossein. Bones are also intersected by 

 blood-vessels, nerves, etc. The marrow of bones consists mainly of fat 

 and albumin. 



The proportion between the organic matter of bone and its mineral 

 constituents is liable to considerable variation, according to the 

 quantities of blood-vessels, nerves, marrow and water which may be 

 present. 



Usually from 30 to 50 per cent of the weight of bone is lost on 

 burning. The composition of similar bones (the metatarsus) of several 

 animals were examined by the writer in connection with an investi- 

 gation of the disease known as "osteoporosis," and the results maybe 

 here given. 



The bones of three healthy mules were found to contain, on the, 

 average 



Moisture 5-34 



Organic matter , 37-77 



Ash 56-89 



10U-00 

 The organic matter contained 



Fat 7-61 



Nitrogen 3-99 



