14 THE FUNCTIONS OF ANIMALS. 



A Sketch of the chief Functions in a higher 

 Organism, such as Man. 



We have seen that the power of movement is one of the 

 chief characteristics of animals, but movement implies a 

 source of energy. This, as is well known, is to be found in 

 food ; therefore we shall begin by studying the way in which 

 food is prepared, so that the tissues of the body may 

 absorb it. 



After the solid food is taken into the mouth, it is cut 

 into small pieces by the teeth, and moistened by saliva, so 

 that it may be easily swallowed. But the saliva has a 

 further function. We eat, in one form or another, consider- 

 able quantities of starch ; but starch occurs as grains of a 

 size that would make it of little use as food if it were not 

 digested. Digestion implies physical and chemical changes 

 which make the solid food-materials soluble, and the saliva 

 has this effect upon the starch, it transforms it into an easily 

 soluble sugar. It does this by means of a ferment, which, 

 however, has never been isolated. The saliva is prepared 

 in three pairs of glands, situated near the mouth. The 

 digestion of starch in the mouth is by no means complete, 

 because it stays there too short a time ; but if a quantity of 

 starch be boiled, so as to soften and break the walls of 

 resistent cellulose that surround the starch granules, and a 

 little saliva added to the mixture, it becomes thin and 

 transparent, and the presence of sugar can be proved by the 

 ordinary chemical tests. In a few minutes a considerable 

 quantity of starch will be transformed into sugar. To what is 

 this power of the saliva due ? If it be filtered, so as to get 

 rid of the mucus and other matters, and mixed with fifteen 

 times its volume of alcohol, a precipitate is formed, which 

 contains all the more complex substances or proteids. In 

 time these coagulate, and become insoluble in water ■ but 

 if the precipitate be now washed, the fluid will be found 

 very active in its starch-transforming powers. So it is neither 

 in the mucus nor in the proteids that the powers of the 

 saliva reside. The active substance is not yet known 

 it has never been isolated, and can be recognised only by 

 its powers. It is called ptyalin ; its peculiarities are : 



