252 The Physiological Role of Saliva 



can be facilitated by sipping fluid frequently; the loss of the pro- 

 tective action of saliva manifests itself, however, in dental caries. 

 Reflecting the different functions of saliva, a great variety of 

 stimuli can elicit salivary secretion. Whereas the unconditioned 

 reflex secretion of gastric juice is evoked essentially by food in the 

 mouth, such secretion of saliva can be produced both by edible and 

 rejectable substances; flow of saliva, contrary to flow of gastric 

 juice, can be elicited not only by agents acting on the taste buds, 

 but by tactile and nociceptive stimuli as well. The remarkable 

 adaptability of salivation to requirements in a seemingly purposeful 

 way has attracted great attention ever since it was first described 

 by Pavlov ; among more recent investigations in this field those by 

 Baxter (1933) and by Montgomery and Stuart (1936) may be men- 

 tioned. Not only is the quantity of saliva varied in an apparently 

 useful way, so that dry material causes a more lively flow than does 

 moist food, and water in itself no flow at all, but the composition 

 of the saliva secreted is well suited to the situation. Edible sub- 

 stances, for instance, cause a secretion of saliva rich in mucin which 

 facilitates swallowing, whereas rejectable agents such as alkali or 

 sand stimulate a lively flow of thin saliva poor in mucin, so-called 

 cleansing saliva. It is easily understandable that the composition of 

 saliva can be varied over a wide range. The saliva produced by one 

 gland, for instance the parotid, differs greatly from that of another, 

 e.g. the sublingual gland, and the contributions of all the glands 

 which together form the mixed saliva may vary considerably with 

 the stimulus. Furthermore, a single gland such as the submaxillary 

 is obviously not uniform but composed of different types of secre- 

 tory cells and their relative contributions to the saliva may vary. 

 Even in a seemingly uniform gland, such as the parotid, the com- 

 position of saliva is greatly variable with the intensity and nature 

 of the stimulus. It is equally comprehensible that these great vari- 

 tions can be brought about from the other end of the reflex arc, the 

 receptors, since the salivary glands can be excited not only from 

 different taste receptors but from touch and pain receptors also. 

 The more precise mechanism through which the adaptation of the 

 secretion to the quality of the stimulus takes place is unknown. 

 The explanation is clearly not to be found in the fact that the gland 

 cells are supplied both with parasympathetic and sympathetic 

 secretory fibres, since the composition of the saliva is adapted to 

 the stimulus even when the sympathetic has been cut. 



