THE MECHANISM OF RESPIRATION 491 



of food supplied under these circumstances, and using the carbohydrates 

 rather than the hydrocarbons. 



Young animals eliminate more carbon dioxide than old ones in pro- 

 portion to their weight, their tissues being more watery, disintegrating 

 more rapidly, and being more amenable to oxidation. Females, on the 

 other hand, making, as a rule, little muscular effort, give off less than 

 males of the same age and weight. 



The proportion of carbon dioxide that is eliminated, as compared with 



the quantity absorbed, is termed the respiratory quotient, and in the horse 



CO CO 



is „ - = 0*9, whilst in carnivora the proportion is — — = 077. 



The amount of water that is expelled from the lungs during respiration 

 is considerable. The average quantity of water contained in air at mean 

 temperature and pressure is about 1 to 1'5 per cent, but the air which 

 is expired is raised to a temperature only a degree or two below 100° F., 

 and it is saturated, containing therefore about 7 per cent of water vapour. 

 It has been estimated that in man about 600 litres of water vapour, weigh- 

 ing 720 grammes, are given off daily, and the quantity given off by a horse 

 may be taken at at least five times that amount. 



THE MECHANISM OF RESPIRATION 



The mechanism by which the respiratory acts are accomplished con- 

 sists of the partly fixed and partly movable bony and cartilaginous frame- 

 work of the chest and of the muscles, which form a large part of the 

 walls of that cavity. The lungs themselves are passive agents, and only 

 contribute to the movement of expiration by their elasticity. They 

 accurately fill and fit the thoracic cavity, except in so far as room is 

 afforded for the heart and great vessels. The principle on which tranquil 

 respiration is accomplished is that the cavity of the chest becomes enlarged 

 by muscular effort. As the diaphragm descends, and as the ribs are raised, 

 air rushes in to equalize the pressure, and inspiration is effected. Then 

 the muscular effort ceases, the diaphragm becomes more arched, the 

 abdominal parietes, chest, and lungs retract by virtue of their elasticity, 

 and expiration results. 



The thorax, or chest, is a conical cavity, narrow and compressed from 

 side to side in front, and expanded behind. It is formed of the strong and 

 unyielding back-bone above, and the sternum, or breast-bone, below. Be- 

 tween these two parts extend the ribs, which number eighteen on each side, 

 the first eight, or true ribs, being attached by their heads or upper ends 

 to the spine, and by their lower ends, with the intervention of a piece 



