THE PHYSIOLOGY OF MUSCULAR WORK 243 



Other factors are probably involved in these conditions ; the 

 contraction of the heart is often irregular during the period of 

 distress, and becomes regular when the " second wind " has arrived. 

 A rise in the temperature of the body, vascular changes and sweat- 

 ing, also accompany " second wind," and further experiments 

 are necessary to determine their significance in relation to the 

 respiratory movements. 



Ryffel has found lactic acid in the sweat and urine after muscular 

 exercise, but no pronouncement can be justly made for or against 

 the theory that lactic acid stimulates the respiratory centre. 



It is known that a rise in the internal temperature ( 43 ) of the 

 body quickens the respiratory movements ; this is especially 

 marked in the dog ; the rate of its respiration may be increased 

 from 28 to 230 per minute when it is necessary for it to cool its 

 body by the evaporation of moisture from its tongue and mouth (**). 



It is probable, moreover, that nervous impulses from various 

 parts of the body, especially from the heart and lungs, may in- 

 fluence the activity of the respiratory centre and produce such 

 a co-ordination of the respiratory movements that an adequate 

 supply of blood is maintained through the lungs. The sensory 

 nerves of muscles may take part, for the respiratory movements 

 are altered in type, rate, and depth by the nature of the exer- 

 cise performed. During rowing a well-trained man adjusts his 

 breathing to his stroke. 



The quantitative changes in the respiratory exchange during 

 exercise will be considered later in connection with the exchange of 

 material, of which they form an essential part. 



INFLUENCE OF MUSCULAR EXERCISE UPON THE TEMPERATURE 



OF THE BODY 



It is necessary here, equally with the other systems of the 

 body, to consider the normal variations in the temperature of the 

 body. One might maintain that it is more necessary, for on ther- 

 mometers and temperature charts the point 98*4 (36'89) has 

 been marked as the normal ; this practice has had the unfortunate 

 result of preventing the full recognition of the daily and personal 

 variations in the temperature of man. Confusion is still greater 

 when it is remembered that many physicians rely upon the tem- 

 perature of the mouth, others upon that of the axilla, and a 



