THE FUNCTIONS OF THE DIGESTIVE ORGANS. 493 



sensitive to acid. Thus the Echinoderms draw in their suckers when 

 exposed to acid. They are then easily removed from the place to which 

 they had attached themselves. 



Let us now return to the saliva. We have already mentioned its most 

 important functions. They are chiefly of a mechanical nature, the 

 food is surrounded by saliva and thus made easy to swallow. The saliva 

 also is an important means of keeping the teeth clean. In this case also 

 it exerts first of all a mechanical action, and, on the other hand, when of 

 normal composition, it also tends to prevent decomposition by the bacterial 

 flora of the mouth. It is probable that in many cases the formation of 

 caries in teeth is due to an abnormal composition of the saliva. Tooth 

 decay, moreover, usually results from a faulty formation of the tooth itself. 

 The tissue composing the teeth is closely related to that of the bones. 

 Three tissues are known to take part in the formation of teeth, of which one, 

 the cement, corresponds to the bony tissue. Dentine also has a similar 

 composition. We find that bones contain: calcium phosphate, magnesium 

 phosphate, calcium fluoride, calcium chloride, calcium carbonate, ferric oxide, 

 and an organic basal substance, which on boiling yields gelatin. Enamel, 

 the third tissue of teeth, is characterized by its containing the least water 

 and the most mineral matter of any substance in the human body. Under 

 normal conditions L exerts great resistance to external influences, and, as 

 long as it is intact, prevents infection by bacteria. 1 The enamel contains 

 lime salts chiefly. At this place we need hardly mention the importance 

 of the food being well chewed. It is perfectly clear that the subdivision 

 of the food is of great benefit as regards the rapidity with which it is acted 

 upon by the digestive ferments. By means of the teeth the food is changed 

 into such a state that it can be acted upon readily by the digestive juices. 

 It might be thought that on feeding with broth, the function of the teeth 

 would be replaced. We shall see later on, however, that the manner in 

 which the food is prepared has a great influence upon the function 

 of the secretions in the digestive tract, especially that of the stomach. 

 A uniform diet could not possibly stimulate our senses permanently. 



In many cases the saliva acts as a solvent, and in this way the sensation 

 of taste is obtained, for we can only taste substances which are in solution. 

 The peripheral organs of the sense of taste are distributed throughout the 

 whole of the mouth. We find them on the upper surface of the tongue, 

 on the under surface at the tip of the tongue, in the mucous membrane of 



1 We should not fail to mention in this case the remarkable readiness with which 

 wounds in the mucous membrane of the mouth, or indeed of the entire alimentary 

 canal, are healed; and how seldom there is any infection here in spite of frequent ex- 

 posure. We may perhaps speak of the immunity of cells in the mucous membrane 

 and surrounding tissue. This immunity may be acquired by the fact that metabolic 

 products of bacteria in the mouth are absorbed from the dilute solution there, and this 

 creates immunity. 



