PESPIRA TION 



139 



now be anything between 70 and 100 per minute. The effect 

 of taking in all this extra air is that the pulmonary ventilation 

 is increased. It is calculated that in man a deep inspiration 

 more than doubles the capacity of the alveoli by distending 

 them. In such paces as the canter, trot, and walk, the amount 

 of air used is correspondingly less ; immediately the pace slackens 

 or the horse stops, the respirations at once fall, and the amount 

 of air inspired becomes reduced. This is one of the great diffi- 

 culties attending respiration experiments on horses under natural 

 conditions. 



In the following table is shown the mean amount of expired 

 air obtained from horses performing actual work, and collected 

 in the apparatus shown in Figs. 52 and 53 : 



A horse in a state of repose, according to Zuntz and Lehmann, 

 produces 85 litres (3 cubic feet) of C0 2 per hour, and absorbs 

 nearly 99 litres (3J- cubic feet) of oxygen ; the expired air is 

 found to have lost 4 per cent, of its oxygen, and gained 3J per 

 cent, of C0 2 . This is very much' more than we found,* 

 but it agrees pretty closely with the observations made on 

 other animals and on man. It may be noted that even in 

 animals which, from their email size or other causes, lend them- 

 selves to exactitude in experimentation, the most divergent 

 results have been obtained, and the same thing is observed 

 in man. 



Muscular work has a profound influence over the respiratory 

 exchange ; it increases the amount of oxygen absorbed and the 

 proportion of carbon dioxide given off. The faster the pace the 

 greater the amount of exchange which occurs, though experi- 

 ment has failed to prove a definitely immediate relationship 

 between the amount of oxygen absorbed and the amount of 

 work produced. 



* ' The Chemistry of Respiration in the Horse during Rest and Work/ 

 Journal -of Physiology, vol. xi., 1890. It is now considered that samples 

 of air are not sufficient to determine respiratory exchanges, the C0 2 has 

 a tendency to accumulate in the tissues, and an apparatus which admits 

 of prolonged observation is necessary, such as was employed by Zuntz 

 and Lehmann. 



