DIGESTION OF FOOD. 319 
point strongly in this direction, though the subject has not yet 
been reduced to scientific form. 
Comparative.—Within the last few years the study of vege- 
table assimilation from the comparative aspect has been fruit- 
ful in results which, together with many other facts of vege- 
table metabolism, show that even plants ranking high in the 
organic plane are not in many of their functions so different 
from animals as has been supposed. It has been known for a 
longer period that certain plants are carnivorous; but it was 
somewhat of a surprise to find, as has been done within the 
past few years, that digestive ferments are widely distributed 
in the vegetable kingdom and are found in many different 
parts of plants. What purpose they may serve in the vege- 
table economy is as yet not well known. At present it would 
seem as though, from their presence in so many cases in the 
seed, they might have something to do with changing the 
cruder forms of nutriment into such as are better adapted for 
the nourishment of the embryo. 
Thus far, then, not only diastase but pepsin, a body with 
action similar to trypsin, and a rennet ferment, rank among 
the vegetable ferments best known. 
A ferment has been extracted from the stem, leaves, and un- 
ripe fruit of Carica papaya, found in the East and West Indies 
and elsewhere, which has a marked proteolytic action. 
It is effective in a neutral, most so in an alkaline medium; 
and, though its action is suspended in a feeble acid menstruum, 
it does not appear to be destroyed under such circumstances, as 
is trypsin. This body is attracting a good deal of attention, 
and its use has been recently introduced into medical practice. 
Very lately also a vegetable rennet has been found in sev- . 
eral species of plants. The subject is highly promising and 
suggestive. . 
SECRETION AS A PHYSIOLOGICAL PROCESS. 
Secretion of the Salivary Glands——We shall treat this subject 
at more length because of the light it throws on the nervous 
phenomena of vital process; and, since the salivary glands have 
been studied more thoroughly and successfully than any other, 
they will receive greater attention. 
The main facts, ascertained experimentally and otherwise, 
are the following: 
Assuming that the student is familiar with the general ana- 
