FORCED BREATBING 39 



dilate, and thus allow the incoming air to gain ready- 

 access to the bronchial tubes and air-cells. 



When the animal is at rest, the elastic recoil of the 

 ribs and lungs is sufficient to expel the iidal air, as the 

 amount changed at each breath is called. The volume of 

 air which remains in the lungs after expiration, greatly 

 exceeds — perhaps five or six times — that of the tidal air. 



In forced breathing, as during active movement, several 

 muscles which are not employed for tranquil respiration 

 are called into play, particularly those used in forced ex- 

 piration, as we may see by the heaving flanks of an 

 animal which is ^' blown/' The muscles that cover the 

 ribs and belly, aid in forced expiration by compressing 

 these parts, and thereby causing the walls of the chest to 

 **fall in,'* and the contents of the abdomen to project 

 forward into that cavity. Quick work naturally develops 

 the muscles of forced respiration. 



I may point out that the muscles of the flank (known 

 to anatomists as the great and small oblique, and the 

 transverse) aid in respiration. Consequently, when they 

 are well developed, the flank is not so hollow as it would 

 be if they were wanting in size. 



I may mention that no hard and fast line can be 

 drawn between tranquil and forced breathing, although 

 the broad differences between them are clearly marked. 



In order that the horse may have his breathing power 

 fully developed, it is necessary that the capacity of the 

 chest at the end of an act of expiration should differ as 

 much as possible from what it will be at the termination 

 of the act of inspiration. For this reason the chest should 

 be deep, the ribs well arched, and, at the same time, 

 they should have a good inclination to the rear. The 

 back ribs should be long, so as to augment the size of 

 the chest ; and the muscles which move the ribs, as well 

 as the muscles of the belly, should be largely developed. 

 I shall further allude, in Chapter XVI L, to the shape of 

 the chest and ribs. 



