342 A MANUAL OF PHYSIOLOGY 



( centre.' It has been shown, however, that this is not a 

 true reflex effect, but is due, mainly at least, to the excitation 

 of certain secretory fibres of the chorda that run for some 

 distance in the lingual, then bend back on their course and 

 pass to the gland. 



The salivary centre can also be inhibited, especially by 

 emotions of a painful kind for instance, the nervousness 

 which often dries up the saliva, as well as the eloquence, of 

 a beginner in public speaking, and the fear which sometimes 

 made the medieval ordeal of the consecrated bread pick out 

 the guilty. 



In rare cases the reflex nervous mechanism that governs 

 the salivary glands appears to completely break down ; and 

 then two opposite conditions may be seen xerostomia, or 

 ' dry mouth/ in which no saliva at all is secreted, and 

 chronic ptyalism, or hydrostomia, where, in the absence of 

 any discoverable cause, the amount of secretion is per- 

 manently increased. Both conditions are more common in 

 women than in men. 



(2) The Influence of Nerves on the Gastric Glands. Like saliva, 

 gastric juice is not secreted continuously, except in animals, 

 such as the rabbit, whose stomachs are never empty. The 

 normal and most efficient stimulus is the presence of food 

 in the stomach. Faintly alkaline liquids, such as saliva, 

 excite an active secretion, but it is only early in digestion, 

 before the reaction of the gastric contents has become 

 distinctly acid, that swallowed saliva can have any effect. 

 Mechanical stimulation of the gastric mucous membrane 

 also causes some secretion, although, it is true, not a great deal. 

 So that at first thought there is much to suggest that the gastric 

 glands are stimulated in a more direct manner than the 

 salivary glands, as by the local action of food substances 

 reaching the cells by a short-cut from the cavity of the 

 stomach, or in a more roundabout way by the blood. And 

 it might be very plausibly argued that the gastric glands are 

 favourably situated for direct stimulation, while the salivary 

 glands are not ; and that the great function of saliva being 

 to aid deglutition, an almost momentary, and at the same 

 time a perilous act, it is necessary to provide by nervous 

 mechanism for an immediate rush of secretion at any instant, 



