RESPIRA TIOX 109 



like a bladder when being distended, but the largest amount 

 of distension occurs where the greatest amount of spongy sub- 

 stance exists. This corresponds to the region where the greatest 

 increase in antero-posterior and side-to-side diameters occurs. 

 Though we have attempted to indicate that some parts of the 

 lung are of more use than others, and that these parts are mainly 

 filled during ordinary respiration, it must not be inferred either 

 that the remaining portions are quiescent or that the lungs are 

 firmly anchored within the chest. As a matter of fact, the roots 

 of the lungs are carried back with each inspiration, and come 

 forward at each expiration, and it is this which explains the up- 

 and-down tracheal movement of the horse in hurried respira- 

 tion. 



A small hole into the pleural sac does not lead to collapse of 

 the lungs even in the horse, but a large opening does. Even 

 then the lungs continue to distend partly and retract, but only 

 with difficulty, and finally asphyxia occurs. In all other animals 

 in which no communication between the pleural sacs exist, the 

 collapse of the one lung under the influence of a large opening 

 into the chest is followed by the opposite lung doing extra work, 

 and even bulging across the middle line of the chest in the 

 mediastinal region. 



In dealing with the distension of the lungs, the condition of 

 repose has been assumed. If work be done, the respirations are 

 increased. They are shorter and shallower, and may vary from 

 20 to 100 or more per minute, depending upon the nature of the 

 work and the fitness of the animal. In such cases the lungs 

 remain more fully distended than during repose, and the actual 

 amount of air entering at each inspiration is less, owing to the 

 fact that the respirations are more numerous, the rule being that 

 when the rate of breathing is increased the depth is decreased. 



Inspiration. — We have seen that during life the lungs occupy 

 the whole cavity of a rigidly air-tight chest. So long as this 

 air-tight condition is maintained, any movement which tends to 

 increase the size of the chest causes distension of the lungs. By 

 so doing, the air within them becomes rarefied, and thus a differ- 

 ence in pressure occurs between the outside air and that already 

 in the lungs. In consequence, air rushes in to restore equilibrium, 

 and this inrush is inspiration. 



An increase in the size of the chest in its two diameters is 

 caused by the movements of the diaphragm and the false ribs. 

 The former retreats and carries with it the abdominal viscera, 

 which are in consequence exposed to compression. The ribs 

 generally are pushed outwards, while above they are drawn 

 forwards, and their posterior edge everted, by which [means the 

 natural curve of the rib increases the side-to-side diameter. At 



