[39] Action of Medicines, 1857. [40] Archiv f. exp. Path. u. Pharm., Bd. VIII, 1878, S. 355. [41] Gazette Médicale, 1873. [42] Journal of Physiology, Vol. XVI, 1882. [43] Arzneimittellehre, Leipzig, 1883. [44] Amer. Journ. Physiol., Vol. I, 1898, p. 411. [45] _Loc. cit._ [46] _Loc. cit._ [47] Amer. Journ. Physiol., Vol. X, 1904, p. 209. [48] In all these experiments there was no interval between the emptying of the loop and the beginning of the 10-minute period which followed. The injections were made as rapidly as possible, and in no case occupied more than a minute. CHAPTER V. The Inhibiting Action of Calcium and Magnesium on the Movements and Secretion of the Intestine. It was first observed by Ringer[49] that the unfavorable effect produced by pure NaCl solution could be lessened by adding other salts, notably calcium and potassium. From this observation there was made the so-called Ringer’s solution, which contains Na, K, and Ca in proportions which render the solution relatively neutral and innocuous towards the living tissues. Howell,[50] working with the heart of the terrapin in various mixtures of Na, K, and Ca chloride, emphasized the importance of calcium in the medium in which the heart beat. He concluded from his experiments that the sodium chloride was mainly instrumental in establishing and maintaining the proper osmotic conditions, while calcium was the main factor in initiating and maintaining the beat of the heart. To quote from his articles:--“The stimulus that leads to a heart contraction is dependent upon the presence of calcium compounds in the liquids of the heart; but for rhythmic contractions and relaxations a certain proportion of potassium compounds is necessary.” “The sodium chloride seems to be essential only in preserving the osmotic relations between the tissues and the surrounding liquid.” Similar conclusions were arrived at by Green.[51]