52 6 THE SALIVAR V GLANDS. 
the How of saliva. Thus, on stimulating the cervical sympathetic in the 
d< >g, 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 Blow 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 scpieezing 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 \ 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." 18S0, Bd. v. (1) p. 7S. 
