CHAP, i.] DIGESTION. 259 



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 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 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 yaso-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. 



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 in- 



. crease 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, and contains few salivary corpuscles 

 or protoplasmic lumps. If the gland itself be watched, while its 

 activity is thus roused, it will be seen that its arteries are dilated, 

 and its capillaries filled, and that the blood flows rapidly through 

 the veins in a full stream and of bright arterial hue, frequently 

 with pulsating movements. If a vein of the gland be opened, this 

 large ^increase of flow, and the lessening of the ordinary deoxy- 

 genation of the blood consequent upon the rapid stream, will be 

 still more evident. It is clear that excitation of the chorda largely 

 Mates the arteries ; the nerve acts energetically as a dilator nerve, 

 probably from acting on some local vaso-motor centre in the gland. 



Thus stimulation of the chorda brings about two events: a 



172 



