mo A MANUAL OF VETERINARY PHYSIOLOGY 



the same time the intercostal spaces widen. By these means 

 the capacity of the chest is increased, and the lungs at once fill 

 the space thus created. 



The tendency of the lungs is to recoil, owing to the highly 

 elastic nature of their tissues, and the fact that they are normally 

 in a state of distension. In the post-mortem condition they 

 are much smaller than the chest. During life the chest cavity, 

 being air-tight and larger than the lungs, the latter are'distended 

 by the atmospheric pressure within them. 



During forced inspiratory movements the elbows are turned 

 out, which brings auxiliary muscles into play, and allows a 

 certain number of true ribs to take part in the chest movement. 



Expiration. — The chest having been filled with air, the next 

 process is its expulsion, and the mechanism concerned here is not 

 entirely agreed upon by physiologists. At one time it was 

 considered to be a purely non-muscular act, the elastic recoil of 

 the lungs and of the abdominal muscles being the important 

 factors ; but if the forward movement of the abdominal contents 

 is to be credited with any influence in expiration, it is evident 

 that the action of the abdominal muscles cannot be of the nature 

 of an elastic recoil. There is some experimental evidence suggest- 

 ing that a reflex nervous co-ordination exists between the 

 inspirating muscles and the abdominal muscles, so that as the 

 former cease to act, the antagonists at once come into operation. 

 It will be shown later that this holds true of the muscles of 

 locomotion, and there seems no reason why it should not apply 

 to the muscles of inspiration and expiration. At any rate, 

 the fact is undoubted that the contents of the abdomen travel 

 chestward, and there seems no great reason why this should 

 be brought about by the elastic recoil of the lung, and of the 

 abdominal muscle, when a purposive movement, for which 

 every facility exists, would be more to the point. As the abdominal 

 contents are forced backwards in inspiration, the gaseous contents 

 of the intestine are submitted to compression. This may be 

 turned to useful advantage during the period of recoil. There 

 is also the elastic recoil of the cartilages of the false ribs, seeking 

 to return to their position of rest after inspiration ; but as 

 material contributions to the effort of expiration, the two elastic 

 recoils are not of any great value. 



Expiration is considered by some physiologists as a passive 

 act, and the view is partly based on the fact that it still con- 

 tinues after all nervous supply to the muscles is cut off. If the 

 cord be divided below the origin of the phrenic nerves, all the 

 muscles of the body posterior to the section are paralysed, 

 but expiration still continues, due to the elastic recoil of the 

 lungs. 



