RESPIRATION. 591 



An experiment first performed by Bernard, showing how an indi- 

 vidual may become accustomed to bad air, fully confirms this statement. 

 He placed under a bell-jar a living sparrow, and, having allowed it to 

 remain there for a certain period, he removed it but slightly influenced 

 by the accumulated carbon dioxide : without replenishing the air, he 

 placed a second sparrow, which had been breathing pure air, under the 

 bell-jar, and death rapidly ensued. He then replaced the first sparrow 

 in the same air, and it also died. 



The influence of age in varying the quantity of carbon dioxide in the 

 expired air is very striking. It has been shown that there is a notable 

 increase in the quantity of carbon dioxide exhaled from infancy to 

 puberty. In the male this quantity continues to increase after puberty 

 until the age of thirty ; from thirty to forty, it remains about stationary ; 

 at forty, it begins to decrease and continues decreasing until the age of 

 sixty, when but little more carbon dioxide is exhaled than by a child of 

 eight years. The same applies, also, to the female. 



Not only does age, but time of day, amount of exercise, character 

 of food, and the temperature also influence the quantity of carbon 

 dioxide exhaled. More carbon dioxide is thrown off in winter than in 

 summer, partly because more food is consumed in winter, and partly, 

 also, because a cubic foot of atmosphere contains a larger quantity of 

 oxygen in winter than in summer, when it has a lower densit}^. During 

 the day more carbon dioxide is thrown off than at night. Alcohol and 

 other articles of the so-called accessory diet diminish the quantity of this 

 gas removed through the lungs, probably by diminishing the waste of the 

 tissues. This matter will be again referred to under the consideration 

 of nutrition. 



The quantity of carbon dioxide eliminated with each expiration is 

 diminished after rapidly repeated inspirations, though the whole quantity 

 exhaled is increased. 



The Respiratory Changes in the Blood.- — As has been already indi- 

 cated, the main points of contrast between the arterial and venous blood 

 consists, in the former in the presence of an excess of oxygen, in the 

 latter of an excess of carbon dioxide, while as the venous blood circulates 

 through the capillaries of the lungs it largely gives up its carbon dioxide 

 and absorbs oxygen. We have now to consider the way in which the 

 gases are held within the blood and the manner in which they enter 

 and leave the blood in the pulmonary and systemic capillaries, and to 

 trace a relationship between these changes in the blood and the changes 

 in the air in respiration. 



When blood is exposed to a vacuum produced by a mercurial air- 

 pump, it will be found to yield about sixty volumes of gas to each one 

 hundred volumes of blood, the temperature being zero C, and the 



