Another, and a more important suggestion has been made by the same chemist. He has pointed out a chemical analogy between certain vegetable compounds and a substance which exists in the bile. He has shown that for the most part the elements of the bile re-enter the blood after passing into the intestine, scarcely more than the colouring matter being finally excreted with the fæces. He has found that if an enema of bile be injected into the rectum, it becomes absorbed there, and does not afterwards pass out into the urine.[33] (_Animal Chemistry_, p. 77.) What the exact function of the bile may be, is as yet undecided. Its most important constituents are, a non-saponifiable fat, Cholesterine; a neutral substance, Taurine; and an acid called Choleic, in combination with Soda,--which latter is present in excess. Bile is bitter to the taste; and ox-gall, or the bile of an ox, has been found, when administered as a medicine, to have an action which resembles that of Tonics. It appears that both the bitterness and the tonic action reside in the Taurine.
M. Liebig has pointed out an important chemical resemblance between this Taurine and the vegetable alkaloids. Of these he has singled out Caffeine, the peculiar principle of Tea and Coffee, as most analogous to the biliary product. From the analysis of these two substances he has deduced the following formulæ.
Taurine = C_{ 4}H_{ 7}NO_{ 10}. Caffeine = C_{ 8}HN_{ 2}O_{ 2}.
Then he shows that an atom of Caffeine, if added to 9 of water and 9 of oxygen, will form two atoms of Taurine. (_Organic Chemistry_, 1842, p. 180.)
It should be remarked that a different formula for Taurine has been arrived at by M. Lehmann and others, who assert that it contains Sulphur. Choleic acid, and other constituents of this complicated fluid, certainly contain sulphur. But even if Sulphur be combined with Taurine, it is not clear that it enters with it into the blood. And the above formula for Taurine has been confirmed by the analysis of M. Löwig.[34] It seems to be sufficiently correct for our purpose.
With the supposed function of Caffeine as a creator of bile, however probable or interesting, we are not now immediately concerned. We are engaged in inquiring whether there is naturally in the blood any substance which resembles the tonic alkaloids. Liebig has shown that this element of the bile is intended for re-absorption into the circulation. And without theorizing at all on the subject, it may readily be demonstrated that, with the existence in the blood of such a material as Taurine, we cannot say that we have not in that fluid an analogue to substances like Quina. Taurine is soluble in water, and crystallizable; is chemically analogous to the tonic bitter principles; and contains the same four elements as Quina. If necessary, this similarity could be exchanged for identity, and Quina could be formed into Taurine in the blood.