5 2 8 THE SA LIVAR J T 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 partially 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 
mucin 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 pepsinogen ; but it is well to remember that there is 
nothing to show that trypsinogen and pepsinogen are less soluble in 
dilute saline solution than trypsin and pepsin ; and further, that there is 
some evidence that the oesophageal 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. 
