Digestive Functions 253 



DIGESTIVE FUNCTIONS 



Saliva is of importance for the mechanical disintegration of the 

 food by mastication, for the transport of the food through the first 

 part of the digestive tract, for dissolving the food, and for its 

 chemical breakdown. 



By moistening dry foodstuffs and the oral mucosa the saliva 

 facilitates chewing. From the food imbibed with saliva a bolus can 

 be formed and rendered slippery by the salivary mucin ; swallowing 

 is furthered thereby. Mucin plays an important role by its lubricant 

 action on the mucosa and the food. 



Water-soluble components of the food can be dissolved in saliva 

 and thus prepared for chemical treatment and eventual absorption. 

 Saliva is, however, of special importance as a solvent because sapid 

 substances can be brought to act on the taste buds only when dis- 

 solved ; therefore, appetite and secretory and motor activity in the 

 alimentary canal are to some extent indirectly dependent on sali- 

 vary secretion. The small salivary glands of the tongue probably 

 play a role in the mechanism of taste; their saliva may remove 

 sapid substances from the taste buds, leaving the receptors pre- 

 pared to receive another stimulus. Some substances are said to give 

 rise to a more persistent taste sensation, for instance iodides and 

 saccharine. This may be due to the fact that they are excreted in 

 the saliva. 



The composition of the saliva itself is of interest in this con- 

 nection. It is remarkable, for instance, that saliva contains no 

 glucose; the blood sugar has to be elevated to a very high level 

 before glucose appears in saliva or adrenaline has to be injected 

 (Hebb and Stavraky, 1936; Langstroth, McRae and Stavraky, 

 1938; Langley, Gunthorpe and Beall, 1958). The absence of 

 glucose in normal saliva must be of significance for the perception 

 of sweet. Similarly, the content of sodium in saliva is low, always 

 considerably below the plasma level (Thaysen, Thorn and 

 Schwartz, 1954). The concentration of potassium of saliva, it is 

 true, is relatively high and often given as approximately four times 

 that of plasma ; but potassium has less effect on the salt receptors 

 than sodium (Ohrwall, 1891). The threshold for salt taste given by 

 most authors seems to be above the sodium concentration of saliva 

 (particularly if the taste buds have not been washed with distilled 

 water before the estimation). Solutions with less sodium than saliva 



