182 ANTHROPOLOGY. 



passages, so that this instrument gives invaluable indications as to the 

 extent and nature of pulmonary disease. 



Physical and chemical phenomena of respiration. The tension of the 

 gases in the air passages of the lungs varies according to the intensity of 

 respiration. In quiet breathing, this amounts to J^ or ^'^ of the strength with 

 which the blood usually flows in the larger arteries. In feeble respiration 

 the amount is not more than half of this. These values increase with any 

 obstruction to respiration. The circumference of the thorax at the pit of 

 the stomach in full grown men, amounts during quiet respiration to from 

 H to \i rarely \^ of the length of the body. Each single aspiration con- 

 tinues longer than a contraction of the heart, and its duration increases 

 with age. The new born infant generally respires forty-four times in a 

 minute, the child of five years only twenty -six. From fifteen to twenty 

 years, twenty times; from twenty to twenty-five, eighteen times; from 

 twenty-five to fifty, sixteen or eighteen times. A mean act of respiration, 

 therefore, in the adult lasts from three to four seconds. These values are 

 of course subject to considerable variation. 



The air introduced into the lungs is first brought to the temperature of 

 the body, whatever be the temperature of the external atmosphere. Again, 

 the air in the lungs is saturated with moisture, the amount dependent upon 

 the barometric pressure and the temperature. During respiration in a cool 

 atmosphere, the exhaled air must contain more moisture than the atmosphere 

 itself Consequently the blood loses more moisture in winter than in 

 summer, this difference, however, being equated by the greater loss by 

 perspiration during the latter season ; in winter again, the amount of urine 

 discharged is greater. The amount of water lost depends npon the size of 

 the lungs. Adult men between seventeen and thirty-five years may lose 

 from ^ to If pounds in twenty-four hours. The number of aspirations does 

 not seem materially to affect the result. 



The composition of atmospheric air is pretty much the same in all 

 countries and during each season of the year, the variation observed being 

 exceedingly slight. Eecent experiments show that there are 20.81 parts 

 of oxygen and 79.19 of nitrogen by volume, and 22.01 of oxygen and 76.99 

 of nitrogen by weight. The amount of carbonic acid appears subject to 

 decided variation, although, under ordinary circumstances, it is exceedingly 

 slight. The air over the sea appears to contain less oxygen than that 

 above the land and along the coast ; and again, on the other hand, the air 

 contained in snow is richer in this gas. According to some observers, the 

 amount of oxygen in the air of high mountains and deep mines is less than 

 the standard. 



A series of carefully conducted analyses of expired air shows that the 

 oxygen is in much less proportion than in the atmosphere, while there is a 

 large amount of carbonic acid. This carbonic acid must therefore have 

 been formed in the system by the combination of oxygen with carbon of 

 the blood. The amount of oxygen absorbed is about 23 per cent. The 

 total amount of carbonic acid exhaled by a man within a given time may 

 be expressed either by weight or volume, or by the value of the carbon 

 888 



