178 EQUINE PHYSIOLOGY. 



In forced inspiration we have coming into action the latissimus 

 dorsi, serratus anticus minor and scaleni. 



In Expiration the air is forced out of the air passages, and is 

 in a passive condition usually, except in forced expiration. 



The Muscles of Expiration are the diaphragm, coming for- 

 ward in a dome-shape, decreasing the antero-posterior diameter 

 of the thorax. 



The internal intercostals, triangularis sterni and serratus pos- 

 ticus minor act by carrying the ribs backward. 



In forced expiration we have coming to action the ilio-spinalis, 

 external and internal oblique, transversalis and rectus of the ab- 

 dominal tunic, by depressing and retracting the posterior ribs. 



Number of Respirations per Minute. 



Ordinary movements, 10 



Walking two hundred yards, 28 



Trotting five minutes, 52 



After five minutes' gallop, 52 



(Colin-Smith.) 



Capacity of the Lungs. 



In man the extreme breathing capacity of the lungs is over 

 three hundred cubic inches, but the ordinary capacity is only 

 about two hundred cubic inches. 



The tidal air is that breathed in and out in tranquil respiration. 



The reserve air is that remaining after an ordinary expiration, 

 but which can be expelled in a full expiration. 



The residual air is that which always remains after a forced 

 expiration. 



The complemental air is that which can be taken into the lung 

 in a forced inspiration. 



The measurements of the capacity of the horse's lungs are unim- 

 portant, but are probably about five or six times as extensive 

 as those of man. 



Changes in the Air in its Passage through the Lungs. 



Atmospheric air consists of 



Nitrogen, 79- l 5 parts. 



Oxygen, 20.81 



Carbon dioxide, 04 " 



