364 COMPARATIVE PHYSIOLOGY. 



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 he observed going on around us and in our- 

 selves. The occurrence of transitional forms, as in the epi- 

 thelium of the digestive 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 enu- 

 meration of details. 



Summary. — A very brief r&sumA 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 such a form that it may be further dealt with by the aliment- 

 ary tract prior to being introduced into the blood (absorption). 

 This is effected in different parts of the tract, the various con- 

 stituents of food being differently modified, according to the 

 secretions there provided, etc. The digestive juices contain 

 essentially ferments which act only under definite conditions of 

 chemical reaction, temperature, etc. 



The changes wrought in the food are the following : starches 

 are converted into sugars; proteids into peptones, and fats into 

 fatty acids, soaps, and emulsion ; which alterations are effected 

 by ptyalin and amylopsin, pepsin and trypsin, and bile and pan- 

 creatic steapsin, respectively. 



Outside the mucous membrane containing the glands are 

 muscular coats, serving to bring about the movements of the 

 food along the digestive tract and to expel the faeces, the circu- 

 lar fibers being the more important. These movements and the 

 processes of secretion and so-called absorption are under the 

 control of the nervous system. 



The preparation of the digestive secretions involves a series 

 of changes in the epithelial cells concerned, which can be dis- 

 tinctly traced, and take place in response to nervous stimula- 

 tion. 



These we regard as inseparably bound up with the healthy 

 life of the cell. To be natural, it must secrete. % 



