346 PRINCIPLES OF GENERAL PHYSIOLOGY 



nerve also contains " trophic fibres." There was no evidence that vaso constrictor 

 fibres were excited in either case. Expressed on Heidenhain's view, we m.iv 

 say that acid excites the " secretory " and vaso-dilator fibres, while the " trophic " 

 fibres are very little affected. Meat excites both the secretory and the trophic 

 fibres of both nerves, in addition to the vaso-dilator fibres of the chorda. 



Bahkin appears to favour the view that the same nerve fibre conveys different kinds of 

 impulses to the cells, hut, as we shall see in Chapter XIII., there are reasons for doubling this. 



Should it be true that there are two different kinds of fibres to the salivary 

 glands, it seems probable that, whenever we have a liquid secretion containing 

 constituents foreign to the blood, the secretion of these substances is under 

 the control of special nerve fibres ; this statement, of course, only refers to 

 those cases where the process is effected by nervous, not by chemical, agency. 



The Pancreas. The chemical mechanism in this case appears to be so adequate 

 and appropriate that its discoverers were inclined to doubt the existence of a 

 nervous mechanism, although we were careful not to deny it (Bayliss and Starling, 

 1902, p. 343). At that time, the experimental evidence did not exclude the 

 possibility of explanation on the lines of a chemical mechanism, but Pavlov has 

 since brought forward evidence which amounts to a satisfactory proof that the 

 vagus nerve contains fibres that cause the production of pancreatic juice, although 

 there are several peculiar facts in connection with the phenomenon. G. von 

 Anrep has demonstrated in England the method of experiment, and there is no 

 doubt that secretion can be obtained by exciting the vagus under certain condi- 

 tions, which have to be pretty closely adhered to. Some reflex inhibitory influence 

 is exercised by the operative procedures, so that it is necessary to divide the 

 spinal cord at the foramen magnum ; the secretion does not appear until after 

 several successive periods of stimulation of the vagus nerve and, when it appears, 

 it is much less copious than after secretin, and contains active trypsin. This last 

 fact presents some difficulty in regarding the vagus effect as a normal mode of 

 production of the juice, since Delezenne and Frouin (1902 and 1903) have shown 

 that the juice which appears copiously from a permanent pancreatic fistula, when 

 food is being digested, both in the dog and in the ox, is inactive until acted upon 

 by enterokinase. The action of the vagus is paralysed by atropine, like other 

 gland nerves. From the very concentrated character of the juice it would seem 

 that the vagus contains chiefly " trophic " fibres. The question of inhibitory 

 nerves to glands will be discussed later. The paper by Bylina (1912) on the two 

 kinds of mechanism, chemical and nervous, should be consulted. 



The view that there are distinct " trophic " fibres in gland nerves receives 

 further support from the changes in microscopic appearances of the gland cells. 

 Excitation of the sympathetic produces considerably more signs of fatigue in the 

 submaxillary gland cells than that of the chorda does. Babkin, Rubashkin, and 

 Savich (1909) have described similar facts with regard to the pancreas. As 

 already stated, by the action of secretin it is difficult to produce signs of fatigue in 

 the cells, while stimulation of the vagus nerve results in marked changes. According 

 to the' observers named, the process of secretion in the case of the chemical 

 excitant is as follows : Water flows through the cell in quantity, and one sees 

 in the cells what look like channels of fluid (see their Fig. 23). This current 

 carries out the zymogen granules into the ducts, where they can sometimes be 

 seen as granules, but they soon become dissolved. It is found, on staining with 

 eosin and orange, that the secretion in the ducts takes the same red colour as the 

 granules inside the cells, and appears to be of the same chemical nature. We 

 know that the trypsin in it is still in the zymogen stage. No cell constituents 

 staining with orange are to be found. 



After nerve stimulation, which gives only a small quantity of a thick juice, we 

 have a different picture. The granules inside the cells undergo a transformation ; 

 they gradually lose the property of staining with eosin or iron haematoxylin and 

 become stainable with orange, sometimes forming large "vacuoles" before passing 

 into the duct. The secretion itself in the ducts stains with orange, not with eosin 

 (see Figs. 16 and 18 of the paper). As we saw, it contains active trypsin. Little 

 or nothing is to be seen of the intracellular channels of the more watery s<><-ivt inn 



