526 THE SALIVAR Y GLANDS. 



the flow of saliva. Thus, on stimulating the cervical sympathetic in the 

 dog, it may happen that the secretion does not begin until the pallor of the 

 gland and the reduction of blood flow are about maximal ; the slow flow 

 of saliva may then continue without change in the blood flow, and may 

 even continue after the end of the stimulation, when the blood vessels 

 are dilated. In the cat, contraction of blood vessels without any flow 

 of saliva can be easily observed by stimulating the sympathetic after 

 about 30 mgrms. of atropine have been injected into the blood. 



2. The quantity of saliva obtained by squeezing the gland is less 

 than that obtained by stimulating the sympathetic. This is most 

 readily observed in the submaxillary gland of the cat, in which about 

 ten times as much saliva is usually obtained by stimulating the sympa- 

 thetic as by squeezing the gland. 



3. The total amount of saliva obtained by stimulating the sympathetic 

 is, in some cases, too great for it to be obtained by simple expression of 

 fluid from the gland. This is perhaps most striking in the case of the 

 augmented secretion of the submaxillary gland of the dog. In favour- 

 able circumstances, \ to J c.c. of saliva may be obtained by a single 

 continuous stimulation, and with a diminution in the size of the gland 

 not appreciably greater than would be accounted for by the diminution 

 in the amount of blood in it. 



Some of these observations, it will be observed, negative also the 

 possibility that the sympathetic saliva can be due to pressure exercised 

 by contractile tissue other than blood vessels around the alveoli. 



We conclude, then, that both the cranial and the sympathetic nerves 

 contain fibres which end in connection with the gland-cells, and which 

 are capable of causing changes in the cells leading to secretion; and 

 we pass on to consider whether the secretory nerve-fibres are of more 

 than one kind. There are two possibilities to take into account : first, 

 whether there are fibres inhibiting the secretion as well as fibres excit- 

 ing the secretion ; and, secondly, whether there are fibres causing 

 chemical changes in the gland distinct from those which cause the flow 

 of fluid. 



The former possibility we may treat briefly. Until it is shown that 

 the decrease in the blood flow through the gland which the sympathetic 

 causes is insufficient to account for the decrease in the flow of saliva 

 which the sympathetic at times produces, this hypothesis of inhibitory 

 fibres does not need serious attention. 



The second possibility we must consider more at length. The 

 theory of the existence of two kinds of nerve-fibres in secretory nerves 

 is due to Heidenhain. 1 



According to this theory, the secretory fibres proper cause certain 

 unknown changes in the cells leading to the passage of fluid through 

 them. The trophic fibres cause chemical changes in the cells leading, 

 on the one hand, to the growth of protoplasm, and, on the other, to the 

 conversion of the stored-up secretory material into a more soluble form. 

 Further, according to this theory, the proportion of these two kinds of 

 nerve-fibres is different in cranial and sympathetic nerves. The cranial 

 nerve contains more secretory than trophic fibres. The sympathetic 

 nerve contains more trophic than secretory fibres. 



The trophic fibres, it will be observed, have two functions, not 

 necessarily connected with one another. The evidence that they cause 



1 Heidenhain, Hermann's "Handbuch," 1880, Bd. v. (1) p. 78. 



