192 HAND-BOOK OF PHYSIOLOGY. 



The elastic force overcome in ordinary inspiration is, according to the 

 same authority, equal to about 170 Ibs. 



Douglas Powell has shown that within the limits of ordinary tranquil 

 respiration, the elastic resilience of the walls of the chest favors inspira- 

 tion; and that it is only in deep inspiration that the ribs and rib-cartilages 

 offer an opposing force to their dilatation. In other words, the elastic 

 resilience of the lungs, at the end of an act of ordinary breathing, has 

 drawn the chest -walls within the limits of their normal degree of expan- 

 sion. Under all circumstances, of course, the elastic tissue of the lungs 

 opposes inspiration, and favors expiration. 



Functions of Muscular Tissue of Lungs. It is possible that the 

 contractile power which the bronchial tubes and air-vesicles possess, by 

 means of their muscular fibres may (1) assist in expiration; but it is more 

 likely that its chief purpose is (2) to regulate and adapt, in some measure, 

 the quantity of air admitted to the lungs, and to each part of them, 

 according to the supply of blood; (3) the muscular tissue contracts upon 

 and gradually expels collections of mucus, which may have accumulated 

 within the tubes, and cannot be ejected by forced expiratory efforts, owing 

 to collapse or other morbid conditions of the portion of lung connected 

 with the obstructed tubes (Gairdner). (4) Apart from any of the before- 

 mentioned functions, the presence of muscular fibre in the walls of a hol- 

 low viscus, such as a lung, is only what might be expected from analogy 

 with o.ther organs. Subject as the lungs are to such great variation in 

 size it might be anticipated that the elastic tissue, which enters so largely 

 into their composition, would be supplemented by the presence of much 

 muscular fibre also. 



EESPIRATOEY CHANGES IK THE Am AKD IK THE BLOOD. 



A. In the Air. 



Composition of the Atmosphere. The atmosphere we breathe has, in 

 every situation in which it has been examined in it* natural state, a nearly 

 uniform composition. It is a mixture of oxygen, nitrogen, carbonic 

 acid, and watery vapor, with, commonly, traces of other gases, as ammonia, 

 sulphuretted hydrogen, etc. Of every 100 volumes of pure atmospheric 

 air, 79 volumes (on an average) consist of nitrogen, the remaining 21 of 

 oxygen. By weight the proportion is N. 75, 0. 25. The proportion of 

 carbonic acid is extremely small; 10,000 volumes of atmospheric air con- 

 tain only about 4 or 5 of carbonic acid. 



The quantity of watery vapor varies greatly according to the temper- 

 ature and other circumstances, but the atmosphere is never without some. 

 In this country, the average quantity of watery vapor in the atmosphere 

 is 1 '40 per cent. 



