CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 399 



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 How ? 



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

 chorda be divided, (lie 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 under the microscope a very 

 few salivary corpuscles, and, occasionally only, amorphous lumps 

 of peculiar material, probably mucous in nature, are seen. If the 

 gland itself be watched, while its activity is thus roused, it will be 

 seen (as we have already said, 167) 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 deoxygenation 

 of the blood consequent upon the rapid stream, will be still more 

 evident. It is clear that excitation of the chorda largely dilates 

 the arteries ; the nerve acts energetically as a vaso-dilator nerve. 



Thus stimulation of the chorda brings about two events : a 

 dilation of the blood vessels of the gland, and a flow of saliva. 

 The question at once arises, Is the latter simply the result of the 

 former or is the flow caused by some direct action on the secreting 

 cells, apart from the increased blood-supply ? In support of the 

 former view we might argue that the activity of the epithelial 

 secreting cell, like that of any other form of protoplasm, is 

 dependent on blood-supply. When the small arteries of the gland 

 dilate, while the pressure in the arteries on the side towards the 

 heart is (as we have previously seen when treating generally of 

 blood-pressure 120) correspondingly diminished, the pressure on 

 the far side in the capillaries and veins is increased ; hence the 

 capillaries become fuller, and more blood passes through them in 

 a given time. From this we might infer that a larger amount of 

 nutritive material would pass away from the capillaries into the 

 surrounding lymph-spaces, and so into the epithelium cells, the 

 result of which would naturally be to quicken the processes going 

 on in the cells, and to stir these up to greater activity. But even 

 admitting all this it does not necessarily follow that the activity 

 thus excited should take on the form of secretion. It is quite 

 possible to conceive that the increased blood-supply should lead 

 only to the accumulation in the cell of the constituents of the 



