(3) The time of peristalsis. When the tube is not being drawn in there is no peristalsis, at least where the balloon is; when the drug acts on the small intestine the peristalsis occurs early; and when on the large intestine, late.
(4) The force of the peristalsis. This can be measured by pulley and bag of shot attached to the graduated tube. Of course it is necessary to learn first the lifting power of the peristalsis in the same dog without a cathartic.
(5) By injecting a solution of the cathartic into a ligatured loop of intestine in the living animal. It is better to use a rabbit or dog that has been starved for several days so that the intestine will be empty. After several hours remove the loop and measure the amount of liquid. If it is increased the drug stimulates secretion. It must not be forgotten that the irritation of the ligature stimulates secretion, so it is better to have a similar loop for comparison.
(6) By injecting an equal quantity of water in two similar loops of intestine in the living animal, and then injecting the cathartic into one of them. Use a starved animal. After several hours remove the loops and measure the quantity of water in each, if the one containing the cathartic has not diminished as much as the other, then it diminishes absorption.
We have now learned how to determine when there is increased peristalsis, and where it is; when there is increased secretion, and when there is diminished absorption. We have next to try to learn the exact manner in which these are produced, but this is far more difficult:
(a) Because the nervous mechanism of the intestines is very complicated, and at present not very much of its physiology is known.
(b) Because instead of the nerves with different functions having a separate course, they are all in the same sheaths, so that one kind of fibres at a time cannot be cut. The vagus contains motor, inhibitory motor, sensory, etc., fibres, so that when one is cut all must be cut. The same is true of the splanchnic.
(c) Because if a drug appears in the urine, sweat, or milk, it is no proof that it acts in the circulation; for it may act locally, then afterwards be absorbed, but have no action on the intestinal mechanism.
(d) Because if a drug cannot be found in the urine, sweat, or milk, it is not proof that it does not act in the circulation, and then afterwards be entirely excreted by the intestinal mucous membrane and pass out with the feces.
