79. The gastric juice is remarkable for three qualities, - a coagulating, antiputrescent, and solvent power. I have already spoken of its coagulating properties. Of its antiseptic powers abundant proofs have been furnished by the experiments of several physiologists. Dr. Fordyce found that the most putrid meat, after remaining a short time in the stomach of a dog, became perfectly sweet. Spallanzani ascertained that the gastric juice of the crow and the dog will preserve veal and mutton, and without loss of weight, for thirty-seven days in winter; whereas, the same meats, immersed in water, emit a fetid smell as early as the seventh day, and by the thirtieth, are resolved into a state of most offensive putridity. The solvent powers of the stomach are equally remarkable. Reaumur and Spallanzani inclosed pieces of the toughest meats, and of the hardest bones, in small perforated tin cases, to guard against the effects of muscular action, and then introduced them into the stomach of a buzzard: the meats were uniformly found diminished to three-fourths of their bulk in the space of twenty-four hours, and reduced to slender threads, and the bones were wholly digested, either upon the first trial, or after a few repetitions.

To ascertain whether the chymification of food were entirely attributable to this gastric solvent, experiments were instituted in order to produce what has been termed artificial digestion. After having macerated food, Spallanzani mixed it with gastric juice, and then exposed it, in a tube, to a temperature equal to that of the stomach; it is said that the experiment succeeded, and that chyme was produced. M. de Mon-tegre, however, has shown the fallacy of this conclusion; but, says M. Majendie, we are not to conclude, from the failure of such an experiment, that the same fluid cannot dissolve the food when it is introduced into the stomach. The circumstances are indeed far from being the same: in the stomach, the temperature is constant, the food is pressed and agitated, and the saliva and gastric juice are constantly renewed; as soon as the chyme is formed it is carried away, - circumstances which do not occur in a tube containing a mixture of the food and gastric juice. It seems probable that the gastric juice remains on the surface of the stomach, and is secreted as the digestion proceeds.

The chynii-fication of the food commences on its surface, and gradually proceeds towards its centre: a soft layer may he easily detached, which presents the appearance of a corroded and half-dissolved substance. The white of a hard egg, for instance, very shortly assumes an appearance like that which would be produced upon it by immersion in vinegar, or an alkaline solution. This change, if duly performed, is not accompanied with any notable extrication of gas; but, should the vital powers of the stomach be deficient, a different species of decomposition takes place, the laws of chemistry gain the ascendancy, and results are produced more or less analogous to those which would arise from the same materials, if placed under similar circumstances of temperature and motion, in a vessel out of the body.

80. Whatever may be the alimentary substance introduced, the chyme will present the invariable property of reddening paper coloured with turnsol, and it has always a sharp odour and taste; vegetables, however, would appear to elicit a more acidulated solvent from the secreting surface of the stomach, than food derived from the animal kingdom.

81. The period necessary for chymification must vary according to the nature and volume of the food, the degree of mastication and insalivation it may have previously undergone, and the degree of vital energy possessed by the stomach. According to the observations of Majendie, fat, tendon, cartilage, coagulated albumen, mucilaginous and saccharine vegetables, resist the action of the stomach longer than fibrinous and glutinous substances. In experiments made by Sir A. Cooper, fat was found to be digested in the stomach of a dog, considerably quicker than muscular flesh, cheese, skin, cartilage, tendon, or bone, each of which had lost less in weight than the preceding, in a given time, through the influence of the gastric solvent. The whole of the aliment is not simultaneously converted, but portions, as they are perfected, are successively passed out of the stomach into the duodenum, there to undergo further changes, to be presently described. In this case the pylorus must, as its name implies, be endowed with a peculiar sensibility and vigilance, by which it is enabled to distinguish between the crude and chymified portions, so as to admit the latter, while it opposes the passage of the former.

To this theory it has been objected, that various foreign bodies have been known to pass from the stomach into the intestines, as buttons, pieces of iron, etc.: but it must be remembered, that such substances may be even less irritating than crude food, and that they are, besides, not admitted into the intestines until they have been frequently presented to the pylorus, and the sensibility of this valve has been diminished. Nature has endowed the eye with an irritability which instantly causes it to close upon the contact of an extraneous substance; but the oculist who is in the habit of performing operations on that organ, knows that, after the instrument has touched the eye several times, its irritability ceases, and it becomes passive. M. Ma-jendie, however, expresses his scepticism with regard to this elective power of the pylorus. He seems to consider the idea as fanciful; but I would ask whether there is anything improbable in the supposition? Is not every part of the machine endowed with a sensibility adapted to the office it is destined to perform? The eye is stimulated by light, the heart by blood; and why may we not suppose that the pylorus is, in a like manner, stimulated by the contact of chyme? If we reject this idea, can we propose a less objectionable explanation of the phenomena? - certainly not: on the contrary, the whole economy of the stomach is adverse to any other belief.

If an unnatural stimulus be given to this viscus, so as to increase its motions, with a view of accelerating the progress of its contents into the duodenum, before they have been duly converted, what happens? The pylorus refuses its assent to their egress, and the motions of the stomach are inverted, so as to expel the crude food by vomiting.

82. In order to facilitate the expulsion of chyme through the pylorus, numerous glands are placed around it, which furnish a lubricating fluid. For the same purpose, glands are found in the crops of birds, near the entrance into the gizzard. In the human subject we recognize a similar provision in the lower part of the rectum, in order to assist the alvine evacuation.