504 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The lungs may be regarded as two large bags broken up into saccular 

 divisions and subdivisions which ultimately consist of a vast number of little 

 pouches, or infundibuli, each of which is, as the name implies, funnel-shaped, 

 the walls being hollowed out into alveoli, or air- vesicles. These alveoli vary 

 in size from 120// to 380 //, the average diameter being about 250 // (y^ inch). 

 Each infundibulum communicates by means of a small air-passage with a 

 bronchiole, which in turn communicates with a smaller air-tube or bronchus, 

 and finally, through successive unions, with the common air-duct or trachea. 

 It is estimated that the alveoli number about 725,000,000, and that the total 

 superficies exposed by them to the gases in the lungs is about 200 square 

 meters, or from one hundred to one hundred and thirty times greater than 

 the surface of the body (1.5 to 2 square meters). The wall of each alveolus 

 forms a delicate partition between the air in the lungs and an intricate net- 

 work of blood-vessels ; this network is so dense that the spaces between the 

 capillaries are, as a rule, smaller than the diameters of the vessels. The 

 lungs, therefore, are exceedingly vascular, and it is estimated that the vessels 

 contain on an average about 1.5 kilograms of blood. Owing to the minute- 

 ness of the capillaries and the density of the network, the air-cells may be 

 said to be surrounded by a film of blood which is about 10 f* in thickness and 

 has an area of about 1 50 square meters. 



The lungs are highly elastic, and their elasticity is perfect, as is shown by 

 the fact that they immediately regain their passive condition as soon as the 

 dilating or distending force has been removed. Before birth the lungs are air- 

 less (atelectatic) and the walls of the bronchioles and the infundibuli are in 

 contact, yet in the child before birth, as in the adult, the lungs are in apposi- 

 tion with the thoracic walls, being separated only by two layers of the pleurae. 

 As soon as the child is born a few respiratory movements are sufficient to 

 inflate them, and thereafter they never regain their atelectatic condition, since 

 after the most complete collapse, such as occurs when the thorax is opened, 

 some air remains in the alveoli, owing to the fact that the walls of the bron- 

 chioles come together before all of the air can escape. As the child grows the 

 thorax increases in size more rapidly than the lungs, and becomes too large, as 

 it were, for the lungs, which, as a consequence, become permanently distended 

 because of their being in an air-tight cavity. If the chest of a cadaver be 

 punctured, the lungs immediately shrink so that a considerable air-space will 

 be formed between them and the walls of the thorax. This collapse is due to 

 the condition of elastic tension which exists from the moment air is introduced 

 into the alveoli, and which increases with the degree of expansion. Therefore, 

 after the lungs are inflated they exhibit a persistent tendency to collapse ; con- 

 sequently they must exercise upon the thoracic walls and diaphragm a constant 

 traction or " pull " which is in proportion to the amount of tension. It is 

 therefore obvious that there must exist within the thorax, under ordinary 

 circumstances, a state of negative pressure (pressure below that of the atmo- 

 sphere). This can be proven by connecting a trocar with a manometer and 

 then forcing the trocar into one of the pleural sacs. 



