CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 335 



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 gas- 

 tric 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 medulla oblongata, apparently not far 

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

 down to the medulla oblongata, that organ being left intact, a flow 

 of saliva may still be obtained by adequate stimulation of various 

 afferent nerves ; when the medulla is destroyed no such action is 

 possible. And a flow of saliva may be produced by direct stimu- 

 lation of the medulla 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 medulla 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 medulla oblongata, 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 

 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 lin- 

 gual, 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. 



189. In life, then, the flow of saliva is brought about by the 



