CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 337 



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 distinctly shew that it need not. 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 immedi- 

 ately 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 gland though the blood-supply is limited to the 

 small quantity still remaining in the blood vessels. 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 ; 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 se :retory fibres, acting directly on the 

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

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

 effect on the latter, paralyses the former just as it paralyses the in- 

 hibitory fibres of the vagus. Hence when the chorda is stimulated, 

 there pass down the nerve, in addition to impulses affecting the 

 blood-supply, impulses affecting directly the secreting cells, and 

 calling them into action, just as similar impulses call into action 

 the contractility of the substance of a muscular fibre. Indeed 

 the two things, secreting activity and contracting activity, are 

 very parallel. 



Since the chorda acts thus directly on the secreting cells, we 

 should expect to find an anatomical connection between the cells 

 and the nerve ; but concerning this our knowledge is as yet im- 

 perfect. 



190. When the cervical sympathetic is stimulated, the 

 vascular effects, as we have already said, 146, are the exact 

 contrary of those seen when the chorda is stimulated. The small 

 arteries are constricted, and a small quantity of dark venous blood 

 escapes by the veins. Sometimes, indeed, the flow through the 



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