422 SECRETION OF SALIVA BY REFLEX ACTION. [BOOK n. 



nerves (ch. t.) towards the ducts of the submaxillary and sublingual 

 glands. It has been much debated whether this ganglion can act as a 

 centre of reflex action in connection with the submaxillary gland, but 

 no conclusive evidence that it does so act has as yet been shewn ; it 

 probably belongs in reality to the sublingual gland. 



Stimulation of the glossopharyngeal is even more effectual 

 than that of the lingual. Probably this indeed is the chief 

 afferent nerve in ordinary secretion. Stimulation of the mucous 

 membrane of the stomach (as by food introduced through a 

 gastric fistula) or of the vagus may also produce a flow of saliva, as 

 indeed may stimulation of the sciatic, and probably of many other 

 afferent nerves. All these cases are instances of reflex action, the 

 cerebro-spinal system acting as a centre. We may further define 

 the centre as a part of the spinal bulb, apparently not far re- 

 moved from the vaso-motor centre. When the brain is removed 

 down to the spinal bulb, that organ being left intact, a flow of saliva 

 may still be obtained by adequate stimulation of various afferent 

 nerves; when the bulb is destroyed no such action is possible. 

 And a flow of saliva may be produced by direct stimulation of the 

 bulb itself. When a flow of saliva is excited by ideas, or by 

 emotions, the nervous processes begin in the higher parts of the 

 brain, and descend thence to the bulb before they give rise to 

 distinctly efferent impulses ; and it would appear that these higher 

 parts of the brain are called into action when a flow of saliva is 

 excited by distinct sensations of taste. 



Considering then the flow of saliva as a reflex act the centre 

 of which lies in the spinal bulb, we may imagine the efferent 

 impulses passing from that centre to the gland either by the" 

 chorda tympani or by the sympathetic nerve. Although it would 

 perhaps be rash to say that in this relation the sympathetic nerve 

 never acts as an efferent channel, as a matter of fact we have no 

 satisfactory experimental evidence that it does so; and we may 

 therefore state that, practically, the chorda tympani is in this 

 action the sole efferent nerve. Section of that nerve, either where 

 the fibres pass from the lingual nerve and the submaxillary 

 ganglion to the gland, or where it runs in the same sheath as the 

 lingual, or in any part of its course from the main facial trunk to 

 the lingual, puts an end, as far as we know, to the possibility of 

 any flow being excited by stimuli applied to the sensory nerves, or 

 to the sentient surfaces of the mouth or of other parts of the 

 body. 



The natural reflex act of secretion may be inhibited, like the 

 reflex action of the vaso-motor nerves, at its centre. Thus when, 

 as in the old rice ordeal, fear parches the mouth, it is probable 

 that the afferent impulses caused by the presence of food in the 

 mouth cease, through emotional inhibition of their reflex centre, to 

 give rise to efferent impulses. 



