448 RESPIRATION. 



The extent of respiratory surface has a very marked in- 

 fluence on the quantity of watery vapor exhaled. This fact 

 is very well shown by a comparison of the exhalation in the 

 adult and in old age, when the extent of respiratory surface 

 is much diminished. Barral found the exhalation in an old 

 man less than half that of the adult. 1 



It is evident that the absolute quantity of vapor exhaled 

 is increased when respiration is accelerated. 



The quantity of water in the blood also exerts an impor- 

 tant influence. Yalentin found that the pulmonary transpira- 

 tion was more than doubled in a man immediately after drink- 

 ing a large quantity of water. 1 



The vapor in the expired air is derived from the entire 

 surface which is traversed in respiration, and not exclusively 

 from the air-cells. The air which passes into the lungs de- 

 rives a certain amount of moisture from the mouth, nares, 

 and trachea. The great vascularity of the mucous membranes 

 in these situations, as well as of the air-cells, and the great 

 number of mucous glands which they contain, serve to keep 

 the respiratory surfaces continually moist. This is important, 

 for only moist membranes allow the free passage of gases, 

 which is of course essential to the process of respiration. 



Exhalation of Ammonia. The most recent and extended 

 observations on the exhalation of ammonia by the lungs, are 

 those of Dr. Richardson, to which we have already alluded 

 in treating of the coagulation of the blood. In more than a 

 thousand experiments, made upon persons of both sexes, and 

 on various of the inferior animals, with but one exception, a 

 notable quantity of ammonia was found in the expired air. 

 Dr. Richardson found the quantity very variable at different 

 times of the day. At certain periods it is absent. 



1 MILNE-EDWARDS, op. cit., p. 625, note. 



9 Ibid., p. 607, note. It has not been thought necessary to discuss the in 

 fluences of dry and moist atmosphere, barometric pressure, and temperature, 

 which are purely physical in their character. 



