In the experiments on the rabbit’s intestine, however, calcium and magnesium have been shown to have an action which can only be described as inhibitory. In an animal as highly organized as the rabbit, the intestine is an extremely complex organ; it is not only a muscular and a glandular organ, but contains a complicated nervous mechanism peculiar to itself which is entirely inseparable from the other parts. It is not possible here to mechanically isolate a part which shall be free from the nervous system as can be done almost completely in the center of the jellyfish and the apex of the heart. We are dealing with an entire organ which must be considered as an indicator by which comparative results may be obtained. So many unknown conditions exist in such an organ that it is impossible to say what tissue is acted on primarily, whether the nervous system on the one hand, or the glandular and muscular tissues on the other. There are, however, two indicators in the intestine by which the comparative actions of substances can be studied, namely, the muscular movements, and the secretion of fluid, both of which by various chemical substances may be increased or lessened; or, in other words, they may be stimulated or inhibited. These terms are entirely comparative. The fact that calcium and magnesium act as inhibitors for both the muscular and secretory activities of the intestine does not imply that they have a similar action in other organs or in all animals. It can only be said with certainty that the chemical conditions under which the intestines of the rabbit live are fairly constant, so that the addition of calcium or magnesium in some way constantly inhibits the activity of both muscular and glandular tissues, and the addition of certain purgative salts constantly stimulates them to greater activity. FOOTNOTES: [49] Journal of Physiology, Vols. 4, 5, 6, 8, 16, 17, and 18, 1883-1895. [50] Amer. Journ. Physiol., Vol. 2, 1898, p. 47. [51] Amer. Journ. Physiol., Vol. 2, 1898, p. 82. [52] Festschrift f. Fick, 1899. Chicago Decennial Publications, 1902, and Pflüger’s Archiv, Bd. 91, S. 248, 1902. Amer. Journ. Physiol., Vol. 5, 1901. [53] Amer. Journ. Physiol., Vol. 4, p. 265, 1900; Vol. 8, p. 75, 1902. [54] Pflüger’s Archiv, Bd. 6, 1872; and Bd. 8, 1874. [55] MACCALLUM, J. B.: Journal exp. Zoology, No. 1, 1905. [56] STOVER, F. H.: Bulletin of Bussey Institution, Vol. II, Part IV, 1884. [57] MACCALLUM, J. B.: Journal Exp. Zoology, Vol. I, No. 1, 1904. [58] MACCALLUM, J. B.: University of California Publications, Physiology, Vol. II, 1905, p. 93. [59] MACCALLUM, J. B.: _Loc. cit._ CHAPTER VI. The Action of Saline Solutions on Loops of Intestine Removed from the Body.