CHAP, i.] DIGESTION. 257 



of food in the mouth may be called forth by an emotion, as when 

 the mouth waters at the sight of food, or by a smell, or by events 

 occurring in the stomach, as in some cases of nausea. Evidently 

 in these cases some nervous mechanism is at work. In studying 

 the action of this nervous mechanism, it will be of advantage to 

 confine our attention at first to the submaxillary gland. 



The submaxillary gland (Fig. 48) is supplied with nerves from 

 two sources : from the cervical sympathetic along the submaxillary 

 arteries, and from the seventh or facial nerve by fibres, which, run- 

 ning in the chorda tympani, join the lingual branch of the fifth 

 nerve, from which they diverge under the lower jaw, and run as a 

 small nerve close beside the duct to the gland. 



If a tube be placed in the duct, it is seen that when sapid 

 substances are placed on the tongue, or the tongue is stimulated 

 in any other way, or the lingual nerve is laid bare and stimulated 

 with an interrupted current, a copious flow of saliva takes place. 

 If the sympathetic be divided, stimulation of the tongue or lingual 

 nerve still produces a flow. But if the small chorda nerve spoken 

 of above be divided, stimulation of the tongue or lingual nerve 

 produces no flow. 



Evidently the flow of saliva is a nervous reflex action, the 

 lingual nerve serving as the channel for the afferent and the small 

 chorda nerve for the efferent impulses. If the trunk of the 

 lingual be divided above the point where the chorda leaves it, as 

 at n. I' Fig. 48, stimulation of the tongue produces, under ordinary 

 circumstances, no flow. This shews that the centre of the reflex 

 action is higher up than the point of section; it lies in fact in 

 the brain. 



In the angle between the lingual and the chorda, where the latter 

 leaves the former to pass to the gland, lies the small submaxillary gan- 

 glion (represented diagrammatically in Fig. 48 sm. gl.) t from which 

 branches pass to the lingual on the one hand and to the chorda on the 

 other ; branches may also be traced towards the ducts and glands and 

 towards the tongue. It has been much debated whether this ganglion 

 can act as a centre of reflex action, but no conclusive evidence that it 

 does so act has as yet been shewn. 



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 also produces 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 



F. 17 



