FORCED BREATHING. 47 



forward by muscles which cover them, and by others which 

 are situated between the successive pairs of ribs. The lower 

 ends of some of the ribs are also drawn up during the act of 

 inspiration. On page 35, I have remarked on the power the 

 ribs have of pivotting on their ends, I may point out that 

 when the air which is within the chest becomes rarefied at the 

 commencement of the act of inspiration, the lungs themselves 

 passively 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 tidal 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 expiration, 

 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 trans- 

 verse) aid in respiration. Consequently, when they are well 

 developed, the flank is not so hollow as it would be if they 

 were weak. 



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. 



