

400 SECRETION AND BLOOD SUPPLY. [BOOK n. 



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 \valls, tend to check that transit. Again, if 

 the head of an animal be rapidly cut off, and the chorda immedi- 

 ately stimulated, a rio\v 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 

 h'bres, one which we may call secretory 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 protoplasm of the se- 

 creting cells, and calling it 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. We know that when a muscle contracts, its 

 blood vessels dilate ; and much in the same way as by atropin the 

 secreting action of the gland may be isolated from the vascular 

 dilation, so (in the frog at all events) by a proper dose of urari 

 muscular contraction may be removed, and leave dilation of the 

 blood vessels as the only effect of stimulating the muscular nerve. 

 In both cases the greater flow of blood may be an adjuvant to, but 

 is not the exciting cause of, the activity of the structures. 



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

 should expect to find an anatomical connection between the cells 

 and the nerve ; and some authors have maintained that the nerve- 



