864 ANIMAL PHYSIOLOGY. 
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 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 
enumeration of details. 
Summary.—A very brief réswmé 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 he 
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 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 
‘pancreatic 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. 
The blood-vessels of the stomach and intestine and the villi 
of the latter receive the digested food for further elaboration 
