(_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. [Illustration: 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. (2) If the drug acts directly upon the cerebral centres, it will act quicker in the animal in which it was injected into carotid artery, because in the other animal it has to be absorbed and diffused through the general circulation before it reaches the centre. Clamping the aorta would make this more accurate were it not that the intestines are so easily disturbed by changing the circulation. [Illustration: FIG. 7.] (3) Exclude the abdominal ganglia. This can be done either by extirpating the ganglia, or by using a piece of excised living intestine. (Fig. 7 represents the abdominal ganglia concerned destroyed.) After extirpating the ganglia, and performing tracheotomy, and injecting the cathartic into the small or large intestine, place the animal in a physiological salt solution in tin vat, and observe the action of the drug. This experiment is not very satisfactory, because the vessels are greatly dilated. (_a_) If the drug does not excite peristalsis, this shows that it acts through the abdominal or cerebral centres. This action may be direct or reflex. (_b_) If the drug excites equally as strong peristalsis as are produced when the ganglia are intact, this shows that its action is not on the abdominal ganglia but on the intestine. (_c_) If the drug excites weaker peristalsis, this shows that it acts at both places. This may be direct or reflex.