COMPOSITION OF THE ATMOSPHERE 285 



in individuals of the height of from five feet seven inches to five feet eight 

 inches, and is equal to a column of three inches of mercury. Above this 

 height the force decreases as the stature increases; so that the average power 

 of men of six feet is measured by about two and a half inches of mercury. 

 The force manifested in the strongest expiratory acts is, on the average, 

 one-third greater than that exercised in inspiration. But this difference is 

 in a great measure due to the power exerted by the elastic reaction of the walls 

 of the chest; and it is also much influenced by the disproportionate strength 

 which the expiratory muscles attain from their being called into use for other 

 purposes than that of simple expiration. The force of the inspiratory act is, 

 therefore, better adapted than that of the expiratory for testing the muscular 

 strength of the body (John Hutchinson). 



It has been shown that within the limits of ordinary tranquil respiration 

 the elastic resilience of the walls of the chest favors inspiration; 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 exhalation has drawn the chest walls within 

 the limits of their normal degree of expansion. Under all circumstances, 

 of course, the elastic tissue of the lungs opposes inspiration and favors 

 expiration. 



It is possible that the contractile power which the bronchial tubes and 

 air- vesicles possess, by means of their muscular fibers may assist in expiration. 

 But it is more likely that its chief purpose is 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. 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 morbid condition of the portion of lung connected with 

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

 functions, the presence of muscular fiber 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 antici- 

 pated that the elastic tissue, which enters so largely into their composition, 

 would be supplemented by the presence of much muscular fiber. 



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, carbon dioxide, 

 and watery vapor, with, commonly, traces of other gases, as argon, ammo- 

 nia, sulphureted hydrogen, etc. Of every 100 volumes of pure atmospheric 

 air, 79 volumes, on an average, consist of nitrogen and argon, the remaining 



