DIGESTION 227 



it. Hence the taking of acid fluids e.g., wines along 

 with starchy food is apt to interfere with the digestion of 

 the latter, although to some extent this is counteracted by 

 the more profuse flow of saliva which acids call out. 

 Tannin is also a powerful inhibitor of ptyalin, which 

 explains part, at least, of the unfavourable effect of 

 strong tea upon digestion. It is interesting to note that 

 the activity of the ptyalin of the saliva is diminished in 

 most cases of dyspepsia, and in dilatation of the stomach 

 it may be entirely absent. On the other hand, in cases 

 of diabetes it is unusually powerful.* 



The secretion of saliva is called out reflexly through 

 the medium of cranial and sympathetic fibres, which are 

 the nerves of secretion. Not all stimuli are capable of 

 exciting secretion in other words, it needs a 'specific' 

 stimulus to do so, but the range of these is very wide. 

 Psychical impressions such as the mere sight or smell 

 of food are powerful stimuli, and may literally ' make 

 the mouth water.' Very dry substances, again, as has 

 already been pointed out, call out a profuse secretion, 

 in order to moisten them and wash them out of the 

 mouth. Acid and acrid substances, too, which, if not 

 neutralized or diluted, would be injurious to the mouth 

 and stomach, strongly excite the glands. In the action 

 of all of these the essentially purposive nature of the 

 salivary reflex is clearly shown forth. Of the two sets 

 of secretory nerves, the cranial fibres produce a more 

 watery flow, and the sympathetic one which is richer 

 in mucin. The latter comes specially into play under 



* 'The Activity of the Saliva in Diseased Conditions of the 

 Body,' Aitchison Eobertson, Journ. of PathoL, 1900, vii. 118. 



152 



