[23] Such is the case in health, when the muscular pressure of the intestine must exceed the pressure on the vascular system on the other side. This latter pressure, when increased, tends to stop or to reverse the inward current. Thus plethora and fever, diseased conditions, counteract absorption by causing pressure on the coats of these capillary vessels. Bleeding,--a drain of fluid caused by an increased secretion,--or such agents as Tartar Emetic and Ipecacuanha,--diminish this pressure by weakening the force of the heart, and thus favour natural absorption. [24] It is stated by Dr. Carpenter, that if a ligature be applied below the pylorus of an animal, and Sulphate of Magnesia then introduced into the stomach, purging is produced. This could only be by the absorption of the salt, and its subsequent action from within on the mucous membrane of the bowel below the ligature. This experiment is unauthenticated, but if verified would certainly be most conclusive in favour of the view adopted above. [25] Hydrochloric acid would probably precipitate medicinal solutions containing silver, and thus render this substance inert when given internally. Another therapeutic argument against this, as the acid of the gastric secretion, may be drawn from the fact that the action of calomel is very different from that of bichloride of mercury,--whereas this acid would convert the former into the latter. [26] Liebig (_Animal Chemistry_, Part I. p. 76) states that the Bile contains Carbonate of Soda. The assertion that the Pancreatic juice is alkaline must be rested mainly on the authority of M. Bernard. [27] Occasionally, as in certain cases of Pyrosis, slight vomiting may take place without any straining effort of this kind, but apparently from the contraction of the stomach alone. [28] Pereira's Materia Medica, 2d edit., vol. ii. pp. 1496, 1507, and elsewhere. [29] Irritant Cathartics, like irritant emetics, may sometimes affect remote parts on the principle of counter-irritation. Some explain in this way the use of purgatives in disorders of the brain, but there are other reasons which serve to account for this still more satisfactorily. (_Vide Chap. IV., Art. Purgatives._) [30] The process of cure by _counter-irritation_ may be ascribed to the _secondary_ action of some medicines. Such an operation may take place at a distance from the part affected; but it is attributable to a peculiar affection of the nervous system, and has no necessary connexion with the primary or proper action of a medicine. _Vide_ p. 67.