DIGESTION OF FOOD. 363 
This is traceable to habit, which is bound up with the law of 
rhythm or periodic increased and diminished activity. 
Indeed, every one sufficiently observant may notice in him- 
self instances of the application of this law in the economy of 
his own digestive organs, 
This tendency i is important in preserving energy for higher 
ends, for such is the result of the operation of this law every- 
where. 
The law of correlation, or mutual dependence, is well illus- . 
trated 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 of 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 under the stress of circumstances. 
It can not be too much impressed on the mind that in the 
complicated 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 modern 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. ig 
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 of fistule 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 
facts of this department of physiology. The structure of the 
