528 THE SALIVAR Y GLANDS, 



case; the more rapidly-flowing fluid might pass more freely into the 

 intracellular spaces, and come into more intimate contact with the 

 mucous or other stored-up material of the cell. 



2. Better evidence of the formation of soluble substances in gland- 

 cells, under the action of nerve stimulation, is afforded by the after-action 

 of strong nerve stimulation. If, between two weak stimulations of a 

 cranial nerve, a strong stimulation of the same nerve or a stimulation 

 of the sympathetic nerve be introduced, the second weak nerve 

 stimulation gives rise to saliva containing a higher percentage of organic 

 substance than that produced by the first similar stimulation. The 

 fact may be taken as showing that the strong stimulation, introduced 

 between the two weak ones, has converted slightly soluble into more 

 soluble material, which has only partTally been carried out of the cell. 

 But this is not the only possible explanation. We can imagine that the 

 stronger the stimulus the more the fluid passing through the cell will 

 be brought into contact with the stored-up substance, with the result 

 that more of this substance will absorb water and pass a stage on the 

 way to solution than would otherwise be the case. And consequently, 

 for some time after a strong stimulus, any fluid passing through the 

 cell would find substance already on the way to solution or already 

 dissolved, without any alteration in its chemical composition. 



The experimental evidence, then, of the formation of a soluble 

 substance during secretion is not satisfactory. And, in fact, it is 

 doubtful whether the glands contain any stored-up organic substance 

 in a " comparatively insoluble " state. The granules of the glands are 

 seen to enter readily into solution micellar or other when a crushed 

 piece of the gland is irrigated with dilute alkaline salt solution. The 

 mucin or mucins of saliva have not been shown to be different from the 

 rnucin or mucins contained in the salivary glands. The mucous material 

 of the glands is often spoken of as mucigen, following the analogy of 

 trypsinogen and pepsiiiogen ; but it is well to remember that there is 

 nothing to show that trypsinogen and pepsiiiogen are less soluble in 

 dilute saline solution than trypsin and pepsin ; and further, that there is 

 some evidence that the cesophageal glands of the frog secrete pepsinogen 

 as such, and not as pepsin. 



Supposing, however, it were shown that nerve stimulation causes an 

 increase in solubility of secretory material, it would still remain to show 

 that this change is caused by a special class of nerve-fibres ; and to this 

 part of the theory we may now pass. 



It was thought that direct proof of the separate existence of trophic 

 fibres was afforded by the results on the parotid gland of stimulating 

 the sympathetic in the dog. Stimulation of the sympathetic caused no 

 flow of saliva, but caused nevertheless histological changes in the 

 gland-cells, and a great increase in the percentage composition of the 

 saliva obtained in other ways. Here was apparently an instance of 

 nerve-fibres producing the changes demanded of the trophic fibres, by 

 hypothesis, and producing no others. 



But we have seen (p. 498) that the sympathetic is capable, in 

 favourable circumstances, of causing a flow of saliva from the parotid 

 gland of the dog. Since, then, secretory fibres are present in the 

 sympathetic strand supplying the parotid, the action of the nerve in 

 this particular instance cannot, without further examination, be taken to 

 show the existence of an additional class of nerve-fibres. 



