(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. [Illustration: 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.