898 ANIMAL PHYSIOLOGY. 
It seems clear that a series of changes constructive and, from 
one point of view, destructive, following the former are con- 
Fic. 279,—Changes in parotid (serous) gland during secretion (after Langley). A, during rest; 
B, after moderate, C, after prolonged stimulation. Figures partly diagrammatic. 
stantly going on in the glands of the digestive organs. Proto- 
plasm under nerve influence constructs a certain substance, 
which is an antecedent of the final product, which we term a 
ferment. It is now customary to speak of these changes as 
constructive (anabolic) and destructive (katabolic), though we 
have already pointed out (page 270) that this view is, at best, 
only one way of looking at the matter, and we doubt if it may 
not be cramping and misleading. 
We must also urge caution in regard to the conception to 
be associated with the use of the terms “ resting” and “ active” 
stage. It is not to be forgotten that strictly in living cells 
there is no absolute rest—such means death; but, if these terms 
be understood as denoting but degrees of activity, they need’ 
not mislead. It is also more than probable that in certain of 
the glands, or in some animals, the processes go on simultane- 
ously: the protoplasm being renewed, the zymogen, or mother- 
ferment, being formed, and the latter converted into actual fer- 
ment, all at the same time. 
It has been pointed out that chorda saliva is usually more 
watery than that secreted under stimulation of the sympathetic. 
When atropine is injected there is no discharge whatever, not- 
withstanding that the usual vascular dilatation follows, from 
which it is clear that the water is actually secreted. 
The nature of secretion is now tolerably clear as a whole; 
though it is to be remembered that this account is but general, 
and that there are many minor differences for each gland and 
variations that can scarcely be denominated minor for different 
animals. Evidently no theory of filtration, no process depend- 
ing solely on blood-pressure, will apply here. And if in this, 
the best-studied case, mechanical theories of vital processes 
utterly fail, why attempt to fasten them upon other glands, as 
