(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.
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.
If it is decided that the drug acts on some part of the intestine to cause the peristalsis, two methods may be used to determine what part it acts upon:
1st Method.—After performing tracheotomy, opening abdomen and destroying abdominal ganglia, place the animal in a tin vat containing warm physiological solution. The first thing is to find out:—
(1) Whether the cathartic acts reflexly by the irritation of the mucous membrane being transmitted to Auerbach’s plexus, and from there to the muscles, or