Page:Popular Science Monthly Volume 35.djvu/827

Rh 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 fæces, the circular 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 distinctly traced, and takes place in response to nervous stimulation.

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 (absorption). The undigested remnant of food and the excretions of the intestine make up the fæces, the latter being expelled by a series of co-ordinated muscular movements essentially reflex in origin.



NE of the most attractive branches of modern chemistry comprises the artificial preparation of compounds existing preformed in nature, or, in other words, the imitation of the works of creative power. Synthesis, as this section of chemical investigation is called, although it has already attained a considerable degree of success, is of but recent origin compared with analysis, or those researches by which we become acquainted with the composition of the products of nature, and of what we derive from them by industrial processes. It is an indispensable condition, before learning how to compound a body, to know what are its constituents, what their properties are, and by what agents they are most liable to be brought into combination with each other. Therefore, synthetical processes could only be founded upon the results of analytical investigations. It is chiefly to the thorough knowledge of the properties and affinities of the seventy so-called elements that we owe the innumerable discoveries which 