EESPIRATION 179 



rapidly to the heart (p. 148), it becomes necessary for the 

 heart to adjust the character of its beat to the new condi- 

 tions ; and this adjustment is the work of the nervous sys- 

 tem. Time is, however, required to make the adjustment, so 

 that it is wise to " warm up " gradually to more vigorous 

 work. We can also understand how by physical training 

 this process of adjustment comes to be shortened, for we 

 have not only trained the heart by giving it more work to 

 do but we have also trained those portions of the nervous 

 system which regulate its beat. 



