SECRETION OF SALIVA AND GASTRIC JUICE. 263 



lying on the small bundles of nerve-fibres which spread out like a fan from the 

 lingual and chorda tympani nerves (ch. t.) toward the ducts of the subm axillary 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 shown ; it probably belongs in reality to 

 the sublingual gland. 



Stimulation of the glosso-pharyngeal 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 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 de- 

 stroyed no such action is possible. And a flow of saliva may be produced 

 by direct stimulation of the medulla itself. When a flow of saliva is ex- 

 cited by ideas, or by emotions, the nervous processes begin in the higher 

 parts of the brain, aud 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 sym- 

 pathetic 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 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 vasomotor 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. 



197. In life, then, the flow of saliva is brought about by the advent to 

 the gland along the chorda tympani of efferent impulses, started chiefly by 

 reflex actions/ The inquiry thus narrows itself to the question: In what 

 manner do these efferent impulses cause the increase of flow ? 



If in a dog a tube be introduced into Wharton's duct, and the chorda be 

 divided, the flow, if any be going on, is from the lack of efferent impulses 

 arrested. On passing an interrupted current through the peripheral portion 

 of the chorda, a copious secretion at once takes place, and the saliva begins 

 to rise rapidly in the tube ; a very short time after the application of the 

 current the flow reaches a maximum which is maintained for some time, and 

 then, if the current be long continued, gradually lessens. If the current be 

 applied for a short time only, the secretion may last for some time after the 

 current has been shut off. * The saliva thus obtained is but slightly viscid, 



