of the internal secretions.

THE HORMONES

The problem of the internal secretions was now attacked from another
angle. A great Russian physiologist, Pawlow, called attention to the
fact that the introduction of a dilute mineral acid, such as the
hydrochloric acid, normally a constituent of the stomach digestive
fluid, into the upper part of the intestine, provoked a secretion
of the pancreas, which is so important for intestinal digestion. He
explained the phenomenon as a reflex, a matter of the nerves going
from the intestine to the pancreas.

His pupil, Popielski, threw doubt upon so easy an explanation, by
proving that the same reaction could be elicited even after all the
nerve connections between the gut and the spinal cord were severed. If
the relation was a reflex, it would have to be classed now as one of
those local nerve circuits, which are pretty common among the viscera,
a local call and reply as it were, without mediation of the great long
distance trunk lines in the spinal cord and the medulla oblongata.

The work of Bayliss and Starling, two English physiologists, was
commenced then to test the hypothesis. They soon found that the
experiment could be so devised as to exclude any influence whatever on
the part of the nervous tissues, and yet result positively. Thus, if a
loop of intestine was so prepared as to be attached to the rest of the
body only by means of its blood vessels, all the nerves being cut,
putting some acid into it was still followed by a flow of pancreatic
juice, no less marked than when none of the parts about the piece
of gut had been disturbed. It was evident that the stimulus to the
pancreas was carried by way of the blood stream. That the stimulating
substance was not the acid itself, was shown by the failure of the
reaction to occur when the acid was injected directly into the blood
stream. Since there was this difference in the effects between acid in
the intestine and acid in the blood, it was manifest that the active
substance must be some material elaborated in the intestinal mucous
membrane under the influence of the acid. So they scraped some of the
lining of the bowel, rubbed it up with acid, and injected the filtered
mixture into the blood. They were rewarded by a flow of pancreatic
juice greater in amount than any obtained in their other experiments.
From the filtered mixture they isolated in an impure form, a solid
substance which, when introduced into the circulation,