THE MECHANICS OF THE RESPIRATORY MOVEMENTS 461 



generally, several days are required before the infantile lung is distended 

 in its entirety. But, the important fact to remember is that the rather 

 sudden increase in the capacity of the thoracic cavity occasions an 

 equally abrupt increase in the volume of the lungs which is made 

 possible only by an influx of air into its passages. In this way, the 

 walls of the different alveoli are put on the stretch and are held in this 

 position throughout the life of the individual. At this very moment 

 arises the elastic recoil of the pulmonary tissue, i.e., the attempt of 

 its constituents to resume their former length and shape. Upon 

 this recoil depends the negative pressure in the thoracic cavity; and 

 clearly, as this difference in the capacity of the thorax and the volume 

 of the lungs is present at the end of expiration and even after death, 

 this negativity is a permanent condition, which may be removed only 

 by perforating the chest wall. It is true, however, that this negative 

 pressure is very slight during the first four days of extra-uterine life, 

 and amounts to only 0.4 mm. Hg at the end of one week. Subse- 

 quent to this time a much greater negativity is gradually developed, 

 because the wall of the chest now becomes more resistant and grows 

 more rapidly than the lung. In order to overcome this difference the 

 lungs are slowly subjected to an even greater expansion, tending to 

 accentuate their elastic tension and to increase the negativity of the 

 intrathoracic pressure. From these facts it may. 1 also be gathered 

 that the lungs of the infant are more thoroughly emptied with each 

 respiratory act than those of the adult. 



B. THE DYNAMIC PHASE 



The Respiratory Movements. The phenomena presented by the 

 active lung, are in no way different from those previously considered. 

 We must remember first of all that the lung remains in firm contact 

 with the chest wall, because its elastic recoil is not sufficient to allow 

 it to separate the visceral from the parietal surface of the pleura and to 

 create a real intrapleural space. Consequently, it must be evident 

 that its degree of expansion is wholly determined by the position of 

 the wall of the thorax. If the latter moves outward, the external 

 surface of the lung must follow in the same direction and give rise to 

 an expansion of the entire organ. Air then rushes into its inner pas- 

 sages. During the succeeding expiratory phase, the chest wall moves 

 inward. The lung then recoils in a certain measure and drives a 

 definite quantity of air to the outside. Obviously, therefore, the lung 

 plays the part of a passive tissue, the active factor being the chest wall. 



The respiratory movements consist of an alternate outward and 

 inward movement of the wall of the thorax which leads first to an 

 increase and then to a decrease in the capacity of this cavity and a 

 corresponding change in the distention of the lungs. The former 

 movement, constituting inspiration, is the result of the activity of the 

 muscles of inspiration, whereas the latter, constituting expiration, is a 



