MECHANISM OF BREATHING. 



51 



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 40, I have remarked on the power the ribs have of pivotting 

 on their ends. When the air which is within the chest, becomes 

 rarefied at the commencement of the act of inspiration, the lungs 

 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 



Laryncc 

 Gullet 



,-Vret^'S 



.Reciiinv 



Bladder 



-Urelhra, 



Fig. 19. — Internal Organs of Horse. 



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 which 

 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 fonvard into that cavity. 

 Quick work naturally developes the muscles of forced respiration. 



The muscles of the flank (known to anatomists as the great and 



4* 



