264 THE TISSUES AND MECHANISMS OF DIGESTION. 



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, 153) 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 lessen- 

 ing of the ordinary deoxygenatiou 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 dilatation of 

 the bloodvessels of the gland, and a flow of saliva. This 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 toward the heart is (as we have 

 previously seen when treating generally of blood-pressure, 109) corre- 

 spondingly 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 epithelial 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 

 saliva, or of the raw materials for their construction, and not to a discharge 

 of the secretion. A man works better for being fed, but feeding does not 

 make him work in the absence of any stimulus. The increased blood-supply, 

 therefore, while favorable to active secretion, need not necessarily bring it 

 about. Moreover, the following facts distinctly show that it need not. 

 When a canula is tied into the duct and the chorda is energetically stimu- 

 lated, the pressure acquired by the saliva accumulated in the canula and in 

 the duct may exceed for the time being the arterial blood-pressure, even that 

 of the carotid artery ; that is to say, the pressure of fluid in the gland out- 

 side the bloodvessels is greater than that of the blood inside the bloodvessels. 

 This must, whatever be the exact mode of transit of nutritive material 

 through the vascular walls, tend to check that transit. Again, if the head 

 of an animal be rapidly cut off, and the chorda immediately stimulated, a 

 flow of saliva takes place far too copious to be accounted for by the empty- 

 ing of the salivary channels through any supposed contraction of their walls. 

 In this case secretion is excited in the gland though the blood-supply is 

 limited to the small quantity still remaining in the bloodvessels. Lastly, if 

 a small quantity of atropine be injected into the veins, stimulation of the 

 chorda produces no secretion of saliva at all, though the dilatation of the 

 bloodvessels takes place as usual ; in spite of the greatly increased blood- 

 supply no secretion at all takes place. These facts prove that the secretory 

 activity is not simply the result of vascular changes, but may be called forth 

 independently ; they further lead us to suppose that the chorda contains two 

 sets of fibres, one which we may call secretory fibres, acting directly on the 

 secreting structures only, and the other vaso-dilator fibres, acting on the 



