THE MECHANICAL CONDITIONS OF RESPIRATION. 275 



In order that the lungs may follow accurately the changes in 

 capacity of the bony cage in which they are enclosed, it is necessary 

 that they should be highly elastic organs ; and we know that the 

 greater part of their framework is made up of a meshwork of elastic 

 fibres. In consequence of this elasticity, the distension of the lungs 

 will require the expenditure of a certain amount of energy, which, 

 as soon as the distending force is removed, will become active and cause 

 collapse of the lungs. In the new-born child, before it has breathed, the 

 lungs are collapsed and airless, and their tendency to contract further is 

 nil. As soon as the first breath has been taken, the cavity of the 

 thorax becomes permanently larger than the volume of the contracted 

 lung, so that, during the whole of life, the lung is in a condition of 

 distension, its collapse being prevented by the resistance of the thoracic 

 wall. This tendency of the lungs to collapse can be shown by piercing 

 the thoracic wall of an animal. As soon as this occurs, the pressures on 

 the outer and inner walls of the lung vesicles become identical ; the lung 

 collapses, and air rushes in through the hole in the pleura to take 

 the place formerly occupied by the distended lung. 



When this experiment is performed on the dead body, the expulsion 

 of air from the lungs is never complete, owing to the fact that the 

 smaller bronchioles collapse and prevent the total emptying of the 

 alveoli by the elastic contraction of the alveolar wall. The elastic 

 reaction of the wall is however not complete until the alveoli are 

 empty. This only happens when in the living animal the inspira- 

 tory force or pull of the thoracic wall is prevented from acting on a 

 portion of the lung, in consequence of the accumulation of fluid in 

 the pleural cavity or obstruction of a bronchus ; in this event the part 

 of the lung that is thrown out of action becomes airless. The air that 

 could not be forced through the bronchioles has been absorbed by the 

 blood circulating through the pulmonary capillaries. 



Since the lungs are in a continual state of tension, they must exert 

 a pull on the thoracic wall and also on all the other structures 

 contained in the thoracic cavity. Hence there is a negative pressure in 

 the pleural cavity. This negative pressure is increased when the volume 

 of the lungs is enlarged, as in a condition of inspiration, and diminished 

 during expiration, but with normal lungs it could only be abolished by 

 a shrinking of the thoracic walls until they enclose a cavity no larger 

 than the atelectatic lung. This negative pressure naturally affects 

 all the other organs enclosed in the thoracic cavity, and therefore 

 exercises an important influence on the flow of blood into and from 

 the heart, as well as on the lymph flow (cf. vol. i. p. 300). 



Inspiration. The inspiratory increase in the thoracic cavity 

 which takes place in all three diameters, is effected by two processes, 

 the elevation of the ribs and the descent of the diaphragm. The 

 articulation of the ribs to the spinal column behind and to the sternum 

 in front allows of a double movement of these bones. Thus, when they 

 are raised, they revolve about an axis passing through the heads of two 

 corresponding ribs, so that the front part of the rib is raised, the angle 

 between rib and costal cartilage straightened out, and the sternum 

 pushed forwards and upwards. At the same time, a slight rotation of 

 the rib occurs, and its outer surface, which in a condition of rest 

 was directed somewhat downwards, is thus caused to look outwards. 

 Moreover, since the transverse diameter of the thorax increases from 



