COMPOSITION OF THE ATMOSPHERE 297 



that the number of the pulses is greater in proportion than that of the 

 respirations. 



The Force of Inspiratory and Expiratory Muscles. The force which 

 the inspiratory muscles are capable of exerting on the chest is greatest 

 in muscular individuals of the mean height of about five feet seven or 

 eight inches and is equal to a column of two and a half to three 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. 



Within the limits of ordinary tranquil respiration the elastic resilience 

 of the walls of the chest favors inspiration. It is only in deep inspiration 

 that the ribs and rib cartilages offer an opposing force to 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 (Gardner). 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 



