8oo THE POPULAR SCIENCE MONTHLY. 



long unable to take food by the mouth, nutritive substances given 

 by enemata find their way up to the duodenum by antiperistalsis. 

 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 com- 

 plicated body of the mammal the work of any one organ is con- 

 stantly 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 attempt to com- 

 pletely unravel vital processes well-nigh hopeless, though each 

 accumulating true observation gives a better insight 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 of fistulce, the condi- 

 tions are so unnatural that any conclusions as to the normal quan- 

 tity from the data they afford must be highly unsatisfactory. 

 Moreover, the quantity must be very variable, according to the 

 law we are now considering. It is well known that dry food pro- 

 vokes a more abundant discharge of saliva, and this is doubtless 

 but one example of many other relations between 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 digestive 

 organs, plainly indicating a rising scale of complexity with greater 

 and greater differentiation of function, is, beyond question, an 

 evidence of evolution. 



The law of natural selection and the law of adaptation, giving 

 rise to new forms, have both operated, we may believe, from what 

 can be observed going on around us and in ourselves. The oc- 

 currence of transitional forms, as in the epithelium of the digest- 

 ive tract of the frog, is also in harmony with the conception of a 

 progressive evolution of structure and function. But the limits 

 of space will not permit of the enumeration of details. 



Summary. — A very brief resume of the subject of digestion 

 will probably suffice. 



Food is either organic or inorganic, and comprises proteids, 

 fats, carbohydrates, salts, and water ; and each of these must enter 

 into the diet of all known animals. They must also be in a form 

 that is digestible. Digestion is the reduction of food to a form 

 such that it may be further dealt with by the alimentary tract 

 prior to being introduced into the blood (absorption). This is 

 effected in different parts of the tract, the various constituents of 

 food being differently modified, according to the secretions there 



