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due to them ; a distinct law of periodicity, various according to the 

 species, is observed in all ; and even what the external conditions do 

 effect, they effect by their influence on processes of cell-life, which 

 they can alter in respect of time in only the same measure, and with 

 the same limitations, as they can alter them in respect of quantity, 

 or any other character. 



Instances of the time-regulation of processes in the animal economy 

 are as evident : witness the returns of thirst and hunger, the regu- 

 lated times for digestion, the rates of excretions, the daily risings 

 and fallings of temperature, the times for the development of ova, for 

 the growth and development of the uterus, and of all the parts apper- 

 taining to gestation and to lactation. 



Equally is time observed in processes of disease. It is most 

 evident, in one sense^ in all intermittent and other periodic affec- 

 tions ; but, in another way, as evident in eruptive fevers. In small- 

 pox, for example, not only is the time set in which the complicated 

 process of eruption will be achieved, but, though every pustule is, in 

 its appearance, independent of the rest, yet all are together in regard 

 of time, because there is the same time-regulation for all. And so, 

 when we watch the progress and the natural remedy of some spread- 

 ing disease, such as erysipelas or gangrene, the observance of time 

 is, in the whole process, as exact as that of quantity, or any other 

 characteristic of the disease. 



I dwell the more on these facts, because, familiar as they are to 

 all, their importance as indicating a general law of organic life seems 

 to be overlooked, and especially their bearing on the explanation of 

 the rhythmic action of the heart has not been duly considered. For 

 not only are many of the processes to which I have referred instances 

 of regular and time-ordered alternations of action, and therefore, in 

 the strict sense, rhythmical ; but they are all instances of that exact 

 observance of minute periods of time, which, on a superficial view, 

 appears as the most singular and inexplicable character of the 

 rhythm of a heart. For, in all these cases, the final correctness of 

 the result of the organic process, the punctuality of its event, is 

 proof that it was exactly timed in every stage ; that it was, through- 

 out, regulated to the second ; just as the action of the heart is, or as 

 the accuracy of a chronometer, at a year's end, is proof, or very 

 nearly proof, that it kept right time in every hour and every second 



