238 THE PHYSIOLOGY OF MUSCULAR WORK 



These results compare very well with those obtained by Haldane 

 and Priestley (^ upon fifteen men ; the means of their results 

 were : 



There is little doubt from the results shown in these two tables 

 that many of the older estimates, which give much lower figures, 

 are incorrect. 



The composition of the alveolar air is not the same in different 

 adult subjects, for FitzGerald and Haldane ( M ) found that the 

 percentage of carbon dioxide varied from 5'86 to 4/29 in men 

 and from 5'40 to 3 - 99 in women. From this the conclusion 

 must be drawn that there is a personal variation in the 

 response of the nervous system to the stimulating action of 

 carbon dioxide. 



The effect of muscular work is always an increased respiratory 

 exchange and increased ventilation of the lungs to meet the greater 

 demand for the supply of oxygen and the removal of carbon dioxide. 

 The breathing becomes quicker or deeper, or more often is affected 

 in both ways ; hyperpncea is produced and, if the work be severe, 

 laboured breathing or dyspnoea may follow and give rise to so much 

 discomfort that the man will pant and stop work to " recover his 

 breath." 



At rest the healthy man breathes through his nose ; the in- 

 spired air is warmed, moistened, and cleansed of foreign particles 

 by passing over the moist surface of the nose and naso-pharynx. 

 During vigorous exercise, however, mouth-breathing constantly 

 replaces nasal breathing, for there is then less resistance to the entry 

 and exit of the air, and the exposure of the moist vascular surface 

 of the mouth assists in the cooling of the body. The man who 

 resists the inclination to breathe through the mouth during active 

 exercise throws an additional and unnecessary amount of work 

 upon his respiratory muscles, and thereby increases his discomfort 

 and distress. 



The increase in the ventilation of the lungs during exercise 



