460 



KESPIRATION 



thoracic pressure apply to one and the same phenomenon, but while the 

 former has reference only to the intrapleural space, the latter includes 

 all regions of the thoracic cavity in which its contents are subjected 

 to the elastic pull of the lungs. The term intrapulmonic pressure is 

 indicative of the pressure prevailing in the air passages of the lungs. 

 At the end of inspiration, as well as at the end of expiration, the pressure 

 in the respiratory channels equals that of the air without. No move- 

 ment of the air is possible as long as the pressures remain equal. 



The Cause of the Negativity of the Intrathoracic Pressure. — During 

 intra-uterine life the lungs are atelectatic, i.e., they contain no air. 

 The walls of their alveoli and smaller tubules are in opposition, while 

 the larger passages contain in all probability a moderate quantity of 

 fluid material. Their solidity leads us to infer a high specific gravity, 



Fig. 236. — Diagram Illustrating the Origin of the Intrathoracic Pressure. 



.A, at birth the lungs contain no air and fill the cavity of the thorax completely; 

 B, after the first respiration the cavity of the thorax is much enlarged in size by the 

 outward movement of the chest walls. Consequently, air is drawn into the recesses of 

 the lung. 



a fact which is employed as a basis for an important medicolegal test. 

 Thus, the lungs of a still-born infant sink when placed in water, while 

 an organ which has been expanded is buoyant and able to carry not 

 only the weight of its own tissue but also that of the heart and large 

 blood-vessels. From this we may draw the conclusion that the lungs 

 of the fetus are under no elastic tension and fill the thoracic cavity 

 completely. Consequently, the capacity of the latter must equal the 

 volume of the lungs, and hence, it must be possible to puncture the 

 chest wall without producing a collapse of these organs. 



At birth, however, the violent muscular efforts immediately give 

 rise to an outward movement of the walls of the thorax and an increase 

 in the size of this cavity. Moreover, as the external surface of the 

 lungs is in firm opposition with the chest wall, it is drawn outward 

 with the result that air now rushes into its innermost passages and 

 recesses. To begin with, only a limited number of alveoli are rendered 

 air-containing, but the succeeding movements distend them in increas- 

 ing numbers until the organ as a whole becomes fully expanded. Most 



