260 ACTION OF CHORDA TYMPANL [BOOK n. 



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 seen in the last chapter 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 must 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 favourable to active 

 secretion, need not necessarily bring it about. Moreover, the 

 following facts are distinctly opposed to such a view. When a 

 cannula is tied into the duct and the chorda is energetically 

 stimulated, the pressure acquired by the saliva accumulated in 

 the cannula 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 outside the blood-vessels is 

 greater than that of the blood inside the blood-vessels. 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 emptying of the salivary channels 

 through any supposed contraction of their walls. In this case 

 secretion is excited in the absence of blood-supply. Lastly, if a 

 small quantity of atropin be injected into the veins, stimulation of 

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

 dilation of the blood-vessels takes place as usual. 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 secreting fibres, acting directly on the epithelium cells only, 

 and the other vaso-motor or dilating fibres, acting on the blood- 

 vessels only, and further that atropin, while it has no effect on 



