RESPIRATION. 1ST 



piration during life ; and Hutchinson calculated, according to the well- 

 known hydrostatic law of equality of pressures (as shown in the Bramah 

 press), that the total force to be overcome by the muscles in the act of 

 inspiring 200 cubic inches of air is more than 450 Ibs. 



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-carti- 

 lages 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 ex- 

 pansion. 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 con- 

 tracts upon and gradually expels collections of mucus, which may have 

 accumulated within the tubes, and which 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 mus- 

 cular fibre in the walls of a hollow viscus, such as a lung, is only what 

 might be expected from analogy with other 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. 



Respiratory Changes in the Air and in 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 its natural state, a 

 nearly uniform composition. It is a mixture of oxygen, nitrogen, car- 

 bonic 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 re- 

 maining 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 contain only about 4 or 5 of carbonic acid. 



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

 ture and other circumstances, but the atmosphere is never without 



