DIGESTION OF POOD. 363 



people; and the same is to a degree true of dogs, etc., kept in 

 confinement, that are taught cleanly habits, and encouraged 

 therein by regular attention to their needs. 



This tendency (rhythm) is important in preserving energy 

 for higher ends, for such is the result of the operation of this 

 law everywhere. 



The law of correlation, or mutual dependence, is well 

 illustrated in the series of organs composing the alimentary 

 tract. 



The condition of the stomach has its counterpart in the 

 rest of the tract ; thus, when St. Martin had a disordered 

 stomach, the epithelium of his tongue showed corresponding 

 changes. 



We have already referred to the fact that one part may do 

 extra work to make up for the deficiencies in another. 



It is confidently asserted of late that, in the case of persons 

 long unable to take food by the mouth, nutritive substances 

 given by enemata find their way up to the duodenum by anti- 

 peristalsis. Here, then, is an example of an acquired adaptive 

 arrangement imder the stress of circumstances. 



It can not be too much impressed on the mind that in the 

 CompKcated body of the mammal the work of any one organ 

 is constantly varying with the changes elsewhere. It is this 

 mutual dependence and adaptation — an old doctrine too much 

 left out of sight in modem physiology — which makes the at- 

 tempt to completely unravel vital processes well-nigh hopeless ; 

 though each accumulating true observation gives a better in- 

 sight into this kaleidoscopic mechanism. 



We have not attempted to make any statements as to the 

 quantity of the various secretions discharged. This is large, 

 doubtless, but much is probably reabsorbed, either altered or 

 unaltered, and used over again. In the case otflstulce, the con- 

 ditions are so unnatural that any conclusions as to the normal 

 quantity from the data they afford must be highly unsatisfac- 

 tory. Moreover, the quantity must be very variable, accord- 

 ing to the law we are now considering. It is well known that 

 dry food provokes a more abundant discharge of saliva, and 

 this is doubtless but one example of many other relations be- 

 tween the character of the food and the quantity of secretion 

 provided. 



Evolution. — We have from time to time either distinctly 

 pointed out or hinted at the evolutionary implications of the 



