RESPIRATION". 271 



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 which cannot 

 be ejected by forced expiratory efforts, owing to collapse or other mor- 

 bid conditions of the portion of lung connected with the obstructed 

 tubes (Gairdner). (4) Apart from any of the before-mentioned func- 

 tions, the presence of muscular 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 Breathed. 



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- 

 bon dioxide, argon, 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. 77, 0. 23. 

 The proportion of carbon dioxide is extremely small; 10,000 volumes of 

 atmospheric air contain only about 4 or 5 of that gas. 



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 at- 

 mosphere is 1.40 per cent. 



Composition of Air which has been breathed. The changes effected 

 by respiration in the atmospheric air are : 1, an increase of temperature; 

 2, an increase in the quantity of carbonic acid; 3, a diminution in the 

 quantity of oxygen; 4, a diminution of volume; 5, an increase in the 

 amount of watery vapor; 6, the addition of a minute amount of organic 

 matter and of free ammonia. 



1. The expired air, heated by its contact with the interior of the 

 lungs, is (at least in most climates) hotter than the inspired air. Its 

 temperature varies between 36 37.5 C. (97 and 99.5 F.), the lower 

 temperature being observed when the air has remained but a short time 

 in the lungs. Whatever may be the temperature of the air when in- 

 haled, it acquires nearly that of the blood before it is expelled from the 

 chest. 



2. The Carbonic dioxide is increased; but the quantity exhaled in a 

 given time is subject to change from various circumstances. From 

 every volume of air inspired, from 4 to 5 per cent of oxygen is abstracted ; 



