750 THE POPULAR SCIENCE MONTHLY. 



and is termed tidal air. Besides the residual air which can not be 

 expelled, the lungs ordinarily contain a large quantity which can be 

 exhaled by a forced expiration. This is known as reserve air. And, 

 in addition to these three kinds, a large quantity can be inhaled by a 

 forced inspiration, called complemented air. 



In this can be seen a fine adaptation to the requirements of varying 

 circumstances ; for, if the breathing of mammals were normally at the 

 full capacity, there would remain no provision for varying the rapidity 

 of respiration according to temperature and exercise. Moreover, the 

 residual air gives uniformity and constancy to the respiratory change 

 of the blood, and prevents sudden variations in the kind, amount, and 

 temperature of the air in the lung-cells, which would be injurious to 

 the blood and to the delicate tissues. The change of the residual air 

 is slowly effected by the physical process of diffusion between it and 

 the reserve air. When this purifying change is not sufficiently rapid, 

 we are impelled to take a " deep breath," and so wholly replace the 

 reserve air. 



The relative and the absolute amounts of each of the four kinds of 

 respiratory air in the human lungs may be tabulated thus : 



f Complemental aii* (can be inhaled by effort). . . 90-110 cubic inches. 



Vital Capacity -l Tidal air (moved in ordinary breathing) 20- 30 " " 



Reserve air (can be exhaled by effort) " 90-110 " " 



200-250 cubic inches. 

 Residual air (can never be exhaled) 100-130 " " 



Total capacity of human lungs 300-380 cubic inches. 



The breathing-pump of mammals may be compared to a conical 

 box with movable sides and base. By contraction of the muscles at- 

 tached to and connecting the ribs, the sides of the chest are moved 

 upward and outward ; while at the same time the diaphragm, forming 

 the arched base of the chest, is depressed or flattened by its muscular 

 contraction. Thus the greater muscular effort in ordinary breathing 

 is used to enlarge the cavity of the thorax or chest, producing inspira- 

 tion not, as in birds, to diminish the cavity, producing expiration. 

 In other words, the air is forced into mammalian lungs by atmospheric 

 pressure when, through muscular effort, the chest enlarges ; and the 

 air is expelled simply by the elastic reaction of the lungs and chest. 



The lungs are freely suspended by the windpipe, and are distended 

 by the atmospheric pressure in opposition to their elasticity. Conse- 

 quently, an opening in the walls of the thorax is liable to produce suf- 

 focation, by giving the air a more direct and easy route to the vacuum 

 of the chest than through the trachea and lungs. 



A delicate membrane called the }?lenra closely invests the lungs, 

 and is then reflected to line the cavity of the thorax. By the secretion 

 of a serous fluid, it prevents friction, which would otherwise result 

 from the constant movement. 



