Fig. 5.
(e) Because when a drug, given hypodermatically, purges, it is not proof that the drug acts as a cathartic in the circulation, and that it does not act locally, for it may be inactive till it is excreted by the intestinal mucous membrane, and then act locally.
At present I will speak of peristalsis only. Perhaps as good a way as any to proceed, though unsatisfactory, is, first, to exclude the brain, and then, second, the abdominal ganglia, and then, third, experiment on the excised living intestine.
(1) Exclude the cerebral centres by cutting the vagi and splanchnics. This is shown in the schematic drawing Fig. 5, in which, for simplicity, one circle represents all the centres in that location. It must not be forgotten that cutting splanchnics affects the intestinal vessels. It is evident that after cutting the vagi and splanchnics the cerebral centres can have no influence on the intestines. After cutting these nerves, give the cathartic:
(a) If it does not act as a cathartic, this shows that it acts upon the intestines entirely through the cerebral centres, which now have no influence upon the intestines. This action is probably direct and not reflex.
(b) If it acts as a cathartic equally as efficient as it does when the nerves are intact, this shows that it does not act as a cathartic through the cerebral centres. The action is either on some of the abdominal ganglia, or on the local intestinal mechanism.
Fig. 6.
(c) If it acts as a cathartic, but much weaker than when the nerves are intact, this shows that it acts both upon the cerebral centres and also on some part of the remaining mechanism. This action on the cerebral centers may be direct, or it may be reflex from irritation of the intestinal mucous membrane. So far as I have read, no experiments have been made to determine when the action is direct and when reflex. It seems probable, though, that something might be learned about it in the following way, which is illustrated by Fig. 6. Select two similar starved animals; open abdomens of both and expose one carotid artery in one of them. Inject a solution of the cathartic in the exposed carotid and an equal solution in the exposed intestinal loop of the other animal. Place animals in salt solution in tin vat so as to observe through incision the intestines.
(1) If the drug acts reflexly by irritation of the intestinal mucous membrane, it will act quicker in the animal in which the drug was injected into the intestinal canal (see Fig. 6), because it is there to begin action at once, while in the other it must first be excreted into the intestinal canal. The intestine can be observed easily in the salt solution.