THE MECHANISM OF EESPIRATION 493 



than might be expected, for if a tape he passed round the animal, just 

 behind the shoulder, it will be found to enlarge only about -g^ inch, at 

 the twelfth intercostal space about 1 inch, and at the last intercostal 

 space about ^ inch. It is estimated that the diaphragm moves back- 

 ward towards the belly about 5 inches, lengthening the chest to that 

 extent. The quantity of air inspired and expired at each respiration varies 

 within wide limits. In tranquil respiration it may amount on the average 

 to about 2 quarts with each respiration, or about 5 gallons per minute, 

 when the number of respirations is ten, but after violent exercise the 

 number of respirations may increase to one hundred or more per minute, 

 with a corresponding increase in the volume of air breathed, amounting 

 in one case to 97 gallons. 



In the horse the act of inspiration is much more prolonged than that 

 of expiration, the proportion being sometimes as much as two to one, and 

 there appears to be a very short interval between the end of the one act 

 and the beginning of the other. 



The air that is used in respiration has been divided into four portions. 

 First, there is that portion taken in during quiet breathing (tidal air), 

 which may be estimated at from 200 to 300 cubic inches. But it is 

 manifest that during exertion the animal can take in an additional amount 

 over and above what it inspires in ordinary breathing. This is termed 

 complemental air. Under these circumstances it also gives out with each 

 expiration much more air than when an expiration has been made during 

 rest; that extra quantity is named supplemental air. And these three 

 portions — tidal, complemental, and supplemental air — are spoken of col- 

 lectively as the "vital capacity". In young and strong animals, with 

 free movement of the chest walls, the vital capacity is large; in old 

 and weakly animals, when the cartilages of the ribs are ossified, and the 

 lungs and thorax are less elastic, it is small. Finally, there is a portion 

 of air termed the " residual air ", which cannot be dislodged by the 

 deepest expiration, but which is still subservient to the respiratory 

 process in the air-cells of the lungs. 



If in a healthy living animal the walls of the chest be cut through, as, 

 for example, by a sabre cut, or be perforated, as by a stake or by a rifle 

 bullet, so that the cavity of the chest is opened, or even if this be done in 

 a dead animal, the lung immediately collapses and retracts fi-om the wall 

 of the chest, and the hissing of air entering through the wound can be 

 distinctly heard. If one side of the lung only be punctured, respiration, 

 though greatly impeded, is yet capable of being performed, but if both 

 cavities are opened snifocation quickly ensues, for with each expansion 

 of the chest the outside air enters the pleural cavity more easily through 



