58 THE MECHANISM OF LIFE 



are thus produced, so that the digestive juices may act all the 

 more easily. These digestive juices play the principal part in the 

 reduction of the food into such a form that it can enter the blood- 

 stream, and we must say something about them. There are 

 certain glands in connection with the alimentary canal (the 

 mouth, oesophagus, stomach, and intestine), and their function 

 is to elaborate liquids which issue from them through ducts, 

 and are conveyed into the cavities of the mouth, stomach, and 

 intestine. The salivary glands in the mouth secrete saliva, the 

 gastric glands of the stomach manufacture an acid liquid called 

 gastric juice, the liver prepares the bile, and the pancreas the 

 pancreatic juice. All along the intestines there are glands 

 which also secrete a digestive juice. 



These various juices are active in virtue of certain mysterious 

 substances called enzymes (or ferments) which they contain. 

 The saliva contains ptyalin, the gastric juice pepsin, the pan- 

 creatic juice trypsin, and the intestinal juices erepsin, etc. The 

 bile secreted by the liver is active mainly in virtue of the alkaline 

 substances that it contains, and the juices of the intestine contain 

 a substance called enterokinase, which " activates " the erepsin. 

 What these enzymes are we do not know in the least, for they 

 have never been prepared in a pure condition, and they are pre- 

 sent in their respective juices in very small quantity. But we 

 know a very great deal about what they do, and there are many 

 inorganic substances which have very much the same properties, 

 so that we cannot say " for certain " that the digestive enzymes 

 are, in any way, substances that are exclusive to the living 

 organism. 



The Foodstuffs. — To understand their modes of operation 

 we must consider the chemical composition of the foodstuffs. 

 Anything that we eat consists of a mixture of substances called 

 proteids, carbohydrates, and fats. White of eggs (albumen) 

 is a nearly pure form of proteid containing much water. The 

 lean flesh of all animals is mainly proteid, while the substance 

 of peas, beans, lentils, cheese, and parts of the cereals and of 

 the solids of milk are proteid in nature. Carbohydrates are 

 represented by cane-sugar and glucose, the starchy foods, such as 

 rice, the substance of potatoes, the main constituents of cereals, 

 the sugar of milk, etc. Fats are the fatty parts of flesh meat and 

 the massive fat that occurs in most animals, the cream of miUc, 

 butter, margarine (which is now mainly " hardened " or " hydro- 



