394 DIGESTION 



easily obtained. After a full meal it cannot be excited at all. 

 When food (or other exciting substance) is repeatedly shown to a 

 fasting animal the reaction becomes each time weaker, and finally 

 the glands cease to respond. All that is then necessary to restore 

 the reaction is to put into the animal's mouth a little of the food 

 (or other object) . When it is now shown it at a distance the ordinary 

 effect follows promptly. This indicates that the condition of the 

 salivary centre exercises an important influence upon the psychical 

 secretion, its excitability to the weaker stimulus set up by the sight 

 of the object being increased by the stronger reflex stimulation 

 coming directly from the mouth. In the condition of satiety the 

 inexcitability of the centre may be due to the action of food- 

 products in the blood. 



In most animals and in man the activity of the large salivary 

 glands is strictly intermittent. But the smaller glands that stud 

 the mucous membrane of the mouth never entirely cease to secrete, 

 and the same is the case with the parotid in ruminant animals. 



The centre is situated in the medulla oblongata, stimulation of 

 which causes a flow of saliva. The chief afferent paths to the 

 salivary centre are the lingual branch of the fifth and the glosso- 

 pharyngeal ; but stimulation of many other nerves may cause reflex 

 secretion of saliva. In experimental reflex stimulation, the sole 

 efferent channel seems to be the cerebral nerve-supply of the glands. 

 After section of the chorda, no reflex secretion by the submaxillary 

 gland can be caused, although the sympathetic remains intact. 



It was alleged by Bernard that, after division of the chordo- 

 lingual, a reflex secretion could be obtained from the submaxillary 

 gland by stimulating the central end of the cut lingual nerve between 

 the so-called submaxillary ganglion and the tongue, the ganglion 

 being supposed to act as ' centre. ' It has been shown, however, that 

 this is not a true reflex effect, but is due to the excitation of certain 

 (recurrent) secretory fibres of the chorda that run for some distance 

 in the lingual, then bend back on their course and pass to the gland. 

 It may be in part a pseudo- or axon-reflex (p. 885), elicited by 

 excitation of efferent fibres, which send branches to some of the 

 ganglion-cells. 



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 hydro- 

 stomia, where, in the absence of any discoverable cause, the amount 



