According to Sollmann (1) and Gaddum (2), castor oil is bland and soothing, but it is split by the lipase in the small intestine, with the consequent liberation of ricinoleic acid, which can stimulate the whole intestine. However, this acid may be absorbed in the small intestine and so fail to reach the colon ; therefore, the cathartic effect of castor oil is due mainly to motor stimulation of the small intestine. The fluid character of the stool is due to the quicker passage of the feces. If the bile is deficient, the oil is not properly emulsified, so the lipase cannot act on it. In such cases, there is no purgation.
The mechanism of the above action was studied with x-ray on the human being. However, researchers (3-6) have said that contrary to the above data ricinoleic acid, applied to the intestinal serosa by Magnus' method in vitro, depresses the movement of the small intestine. Yamamoto (5) has surmised from this strange phenomenon that the ricinoleic acid liberated by the lipase in the small intestine may be absorbed in the blood stream and stimulate the excitability of the Plexus Auerbachii lowered under the influence of bile, although ricinoleic acid alone can depress it.
Furthermore, Nakaya (7) has supposed that the ricinoleic acid may stimulate both the Plexus Auerbachii and the vagus center. However, neither Yamamoto nor Nakaya explained how the ricinoleic acid alone was absorbed in the small intestine during purgation without any absorption of water, or why the ricinoleic acid was not changed to sodium ricinoleate in the alkaline content of the small intestine. Moreover, if the bile should lower the excitability of the intestine, then the normal intestine, under the influence of bile, would have no tonus, and would relax completely. While the ricinoleic acid stimulates the vagus center, the alimentary tract may be stimulated in all areas. In spite of these facts, Yamamoto and Nakaya have claimed that the small intestine alone is stimulated.
Thus the literature on this subject presents conflicting results. In view of this fact, the following experiments were carried out to clarify these questions.