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MESSRS. J. N. LANGLEY AND H. IL FLETCHER 
tlie percentag'e of salts in saliva increases with its rate of secretion. As we shall 
frequently have occasion to refer to this conclusion, we shall, for the sake of brevity, 
call it Heidenhain’s law. 
Since, in experiments like those of Ludwig and Becher, the rate of flow of saliva 
would, as a rule, steadily decrease, it was most probable that the variations in the 
percentage of salts observed by them were due to variations in the rate of secretion. 
And Heidenhain* * * § came to the conclusion that the percentage of salts in saliva was 
not influenced by tbe state of the gland, except in so far as the state of the gland led 
to an alteration in the rate of flow ; so that at the end of a protracted secretion, the 
percentage of salts would be the same as at the beginning, provided the rate of 
secretion were the same. 
Werther,! in the course of some observations on the secretion of the various salts 
which occur in saliva, has repeated Heidenhain’s experiments, taking larger quan¬ 
tities of saliva, and confirms Heidenhain’s conclusions. 
So far, then, it would appear that the secretion of salts depends in some not 
clearly definable way upon the secretion of water, and upon that alone.;}; 
Both in Heidenhain’s and in Werther’s Tables there are a considerable number 
of departures from the law that an increased rate of secretion causes an increased 
percentage of salts. In Heidenhain’s^ experiments, out of thu’ty-six estimations 
there are thirteen divergences from the law. Some of these, it is true, are slight. 
They are all referred by Heidenhain to unavoidable variations in the rate of secretion 
during the time of collecting each sample of saliva. But it must be noticed that 
Heidenhain does not expressly say that he observed during the collection of the 
samples of saliva any especial variation in the rate of secretion of those particular 
samples which, on analysis, were found not to follow the laAv of increased percentage 
of salts with increased rate of secretion. Hence, although the explanation is a 
probable one, it is, as matters stand, not satisfactorily proved. 
Causes of Variation in the Rate of Secretion, on apparently Equal Stimulation of 
the Chorda Tympani. 
In order to observe accurately the connection between the rate of secretion and the 
percentage of salts in the saliva, it is essential that each sample of saliva should be 
secreted at the same rate throughout. It is, however, impossible, except in very 
large Dogs, to obtain a sufficient quantity of saliva for analysis, the rate of secretion 
* Heidenhain, op. cit., 1878. 
t Werther, ‘ Ai'chiv f. d. ges. Physiologie,’ vol. 38, 1886, p. 293. 
t It was some time ago pointed out by one of us (‘ Journal of Physiology,’ vol. 2, p. 269, 1879) that 
the percentage of salts in saliva does not always increase with an increase in the rate of secretion. But 
the only analyses given were of parotid saliva in the Dog, obtained first by stimulating the sympathetic, 
and then by injecting pilocarpin. For an account of the recent observations of Novi, cf. p. 150. 
§ ‘ Archiv f. d. ges. Physiologic,’ vol. 17, p. 8, Table IL 
