There are two other properties of the gastric juice, besides its solvent power, which are at least as difficult to account for, but of which we seem to have very complete evidence,— its property of coagulating albumen, and that of preventing putrefaction. It is the former of these properties which we employ ii, mak ing cheese, cheese being essentially the albu minous part of milk, coagulated by means of what is termed rennet, a fluid consisting of the infusion of the digestive stomach of the calf. This is unequivocally a chemical change, yet it is very difficult to explain it upon any che mical principle, i. e. to refer this individual case to any series of facts, with which it can be connected.* 'We can only say in this instance, as in so many others in the physical sciences, that although the fact is clearly ascertained, its efficient cause still remains doubtful.
We are compelled to make the same re mark with regard to the other property of the gastric juice, to which we have referred above, its antiseptic power. Of the fact, however, we are well assured, both as occurring in the natural process of digestion, and in the expe riments that have been made out of the body. It is not uncommon for carnivorous animals to take their food in a half putrid state, when it is found that the first action of the gastric juice is to remove the fcetor; and an effect of precisely the same kind was noticed by Spal lanzani in his experiments.t IIere again we have a chemical change, the nature of which we cannot explain; it is, however, a circum stance which may appear less remarkable, with respect to the subject now under consideration, because the action of antiseptics generally is one which we find it difficult to refer to any general principles.
Respecting the process of chymification it only remains for us to remark, that the con tractile action of the stomach is admirably fitted to aid the chemical action of the secreted fluids ; the vermicular motion of the organ has the effect of keeping the whole of its contents in a gradual state of progression from the cardia to the pylorus, while, at the same time, each individual portion of the aliment is com pletely mixed together, and brought into the proper state for being received into the duode num. The undulatory motion of the stomach is more especially effected by the circular fibres, while the longitudinal fibres are more effective in the progressive motion of its contents from the cardia to the pylorus.
The alimentary mass is now to undergo the last of the three changes to which we referred above, its conversion from chyme into chyle. These substances are obviously different from each other in their sensible properties, but respecting the exact nature of this difference, the change which they experience, or the mode in which it is produced, we have little certain information. The fact appears to be, that as soon as the uniform pultaccous mass, which composes the chyme, enters the duodenum, it begins to separate into two parts, a white creamy substance, which constitutes the chyle, and a residuary mass, which is gradually con verted into faces, and is propelled along the course of the intestine, in order to be finally expelled from the system.* Although no point in physiology appears to be more clearly as certained than that chyle, properly so called, is never found in the stomach, and that the duodenum is the appropriate organ for its pro duction, yet owing partly to the inaccurate mode in which the terms have been employed, and partly to the inaccuracy of our obser vation, some writers, even in our own times,} have spoken of chyle as being formed in the stomach, and have conceived that the only change which was effected in the duodenum was the separation of the chyle from the re mainder of the mass.! With respect to the mode in which this change is brought about, or the agent by which it is effected, we have little to ofler except con jecture. The secretions of the liver and the pancreas are, each of them, conveyed into the duodenum, and it has been stated that the completion of the chyle takes place exactly at the part where the bile and thepancreatic juice enter into the intestines. Of this, however,
we do not possess any direct evidence, and the fact, that in certain cases of disease or mal formation, the process of chylification has gone on, nearly in its ordinary course, although the fluids in question have not been transmitted into the intestineh appears to furnish a de cisive objection to the hypothesis. Some phy siologists have conceived that the duodenum itself secretes a specific fluid, analogous to that in the stomach, by which the process of chylification is effected ; but we have no evi dence of the existence of this fluid, except the supposed necessity to explain the effects that are produced. In this deficiency of direct evi dence we appear to be reduced to the sup position, that the conversion of chyme into chyle is effected partly by the mutual action of its constituent elements on each other, aided perhaps, in some degree, by the intervention of the bile and the pancreatic juice.* We have various analyses of chyle, which appear to have been made with sufficient accu racy. It is a white opaque substance, re sembling cream in its appearance and phy sical properties. When removed from the body, it shows a tendency to concrete and undergoes a change considerably resembling the coacrulation of the blood, by which it se ? parates into two parts, a dense white coagulum, and a transparent colourless fluid, analogous respectively to crassamentum and to serum. The chemical properties. of chyle appear very similar to those of the blood, and it also re sembles blood in the nature of its saline con tents ; but it differs from it in containing a portion of oil as one of its essential consti tuents, while in the blood oil is only an occa sional, and probably a morbid ingredient:I The chemical analysis of chyle was first made by Vauquelin, who employed for this purpose the contents of the thoracic duct and large lacteals of a horse. The coagulum from the duct was observed to be of a light pink colour, while the corresponding part from the lacteals was nearly white; but it is not ascer tained how far this difference of colour de pended upon an accidental occurrence, or whether it is to be regarded as a uniform cir cumstance. The coagulum contained a sub stance which bore a considerable resemblance to fibrine, or perhaps more correctly possessed properties intermediate between fibrine and albumen. The liquid part of the chyle was found to be very similar to the serum of the blood, differing from it only in containing a quantity of an oily or fatty substance; like serum it exhibited marks of an uncombined alcali.t The next experiments which we possess are those of Maim, who operated upon the chyle as procured from dogs. One main object of his researches was to ascertain how far chyle of animal origin differs from that from vege tables, and he had the food of the dogs regu lated accordingly. His results with regard to the general nature and properties of chyle cor respond very exactly with those of Vauquelin. Ile found the coagulum to have a pink colour, and to contain a fibrous or filamentous sub stance, while the liquid part contained a quan tity of an oily matter, which floated on its surface like cream. This oily matter appeared, however, to be confined to the animal ehyle, and it is remarked generally, that this bore more resemblance to blood than the chyle from vegetables. They contained the same saline ingredients, but the solid residuum of the animal chyle was considerably greater; and as the vegetable chyle, when submitted to destructive distillation, was found to contain much more carbon, it was inferred that the animal chyle must have contained proportion ably more hydrogen and nitrogen.* Upon these experiments we may remark, that the difference between the animal and the vege table chyle in this case might perhaps depend in some degree upon vegetable food being less adapted to the digestive organs of the dog ; because the chyle of the horse, as examined by Vauquelin, appeared to be more completely animalized, although it must have been derived from vegetable diet.