RESPIRATION. 



345 



nected with the change in the relation of the lungs to the thorax 

 attending the first inspiration. Previous to birth the walls of the 

 alveoli and bronchioles are collapsed and in apposition. The larger 

 bronchial tubes in all probability contain fluid. The lungs therefore 

 are devoid of air (atelectatic), and, having a specific gravity greater 

 than water, readily sink when placed in this fluid. The capacity 

 of the thorax does not exceed the volume of the lungs. With the first 

 inspiration, however, the thoracic walls take a new position. The 

 air at once rushes into the lungs and distends them. But as the 

 capacity of the thorax even at the end of the expiration is now greater 



M. 



FIG. 1 66. SECTION OF THORAX WITH THE LUNGS, HEART, AND PRINCIPAL VESSELS. 

 5". Catheter introduced into the pleural space and connected with a manometer. 

 (After Moral and Doyen.} 



than the volume which the lungs could assume without consider- 

 able distention, there at once arises the elastic recoil in the opposite 

 direction, the condition which gives rise to the negativity of the 

 pressure in the thoracic cavity. It is also probable that as the child 

 develops, the thorax grows more rapidly than the lungs, giving 

 rise to a condition which would increase and accentuate the elastic 

 tension and thus increase the negativity of the intra-thoracic pressure. 

 Dynamic Relations. In the dynamic conditions as previously 

 stated these relations change. The intr a- pulmonary pressure varies 

 during both inspiration and expiration. With the enlargement 



