THE CAUSATION OF DISEASE. 5 I 



crying out for more." This, no doubt, is one' element in 

 the causation of the recurring hunger, but the sensation 

 tends to return at the wonted hour altogether indepen- 

 dently of the time of the previous meal, nay, even though 

 the customary previous meal has been omitted ; moreover, the 

 sensation of hunger will often disappear if the meal be not 

 taken at the proper hour, just as the sleepiness at the wonted 

 bedtime may pass off if the individual, through force of will or 

 through necessity, keep himself awake beyond this time. A girl 

 of fifteen, for instance, sits up for several nights in succession with 

 her mother ; at the usual bedtime the heaviness of sleep comes 

 over her, but it quite passes off after she has struggled against 

 it' for half an hour or so. The same principle applies to 

 the practice of smoking. A man, accustomed to smoke at a 

 particular hour, will experience the desire to indulge his habit 

 at that particular hour, and generally at that hour only. Now 

 all the above phenomena are due to rhythmical changes im- 

 pressed upon the system by subjecting the body to certain other 

 rhythmical changes. Each and all are distinct rhythms, the 

 interval being, for the most part, twenty-four hours. Thus, 

 in the case of the ordinary sleep rhythm, at the end of each 

 twenty-four hours the body passes through a particular change. 

 The molecules of the brain, namely, fall into a material state 

 which expresses itself psychically as a desire for sleep. In like 

 manner the digestive organs and that part of the sensorium 

 which has to do with appetite are thrown into rhythmical 

 change, and hence the rhythmical desire for food. 



So much for the bodily rhythms connected with the daily 

 rotation of the earth on its axis. The yearly movement of the 

 earth round the sun leads in like manner to rhythmical organic 

 changes. These are especially noticeable in the plant world, 

 but they are likewise very marked among animals. Thus, many 

 animals change their abode at stated seasons. A large number, 

 again, hybernate. In others, marked dermal changes take place 

 at particular seasons, such as the casting of the shell in certain 

 Crustacea, the shedding of the horns and of the coat in many 

 quadrupeds, and the moulting of birds — not to mention the 

 extraordinary tegumental changes which take place in a large 

 number of animals during the season of sexual activity. 



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