DIGESTION OF FOOD. 329 
the kidneys and the lungs, or, indeed, apply such crude concep- 
tions to the subtle processes of living protoplasm anywhere or 
in any form ? 
It is somewhat remarkable that an extract of a perfectly 
fresh pancreas is not proteolytic; yet the gland yields such an 
extract when it has stood some hours or been treated with a 
weak acid. These facts, together with the microscopic appear- 
ances, suggested that there is formed a forerunner to the actual 
ferment—a zymogen, or mother-ferment, which at the moment 
of discharge of the completed secretion is converted into the 
actual ferment. We might, therefore, speak of a pepsinogen, 
typsinogen, etc., and, though there may be a cessation in the 
series of processes, and no doubt there is in some animals, this 
may not be the case in all or in all glands. 
Secretion by the Stomach.—The glands of the stomach differ 
in most animals in the cardiac and pyloric regions. In those 
of the former zone, both central, columnar, and parietal (ovoid) 
cells are to be recognized. It was thought that possibly the 
latter were concerned in the secretion of the acid of the 
stomach, but this is by no means certain. Possibly these, like 
the demilune cells of the pancreas, may be the progenitors of 
the central (chief) cells. The latter certainly secrete pepsin, 
and probably also rennet. Mucus is secreted by the cells lining 
the neck of glands and covering the mucous membrane inter- 
vening between their mouths. The production of hydrochloric 
acid by any act of secretion is not believed in by all writers, 
some holding that it is derived from decomposition of sodium 
chloride, possibly by lactic acid. So simple an origin is not 
probable, not being in keeping with what we know of chemical 
processes within the animal body. 
Self-Digestion of the Digestive Organs,—It has been found, both 
in man and other mammals, that when death follows in a 
healthy subject while gastric digestion is in active progress 
and the body is kept warm, a part of the stomach itself and 
often adjacent organs are digested, and the question is con- 
stantly being raised, Why does not the stomach digest itself 
during life? To this it has been answered that the gastric 
juice is constantly being neutralized by the alkaline blood; 
and, again, that the very vitality of a tissue gives it the neces- 
sary resisting powers, a view contradicted by an experiment 
’ which is conclusive. If the legs of a living frog be allowed to 
hang against the inner walls of the stomach of a mammal 
when gastric digestion is going on, they will be digested. 
