88. These observations are of great value to the physiologist, as well as to the pathologist, as they demonstrate the fallacy of that proposition which has been so frequently advanced; viz. "that there are many species of food, but only one aliment;" intimating thereby, that all substances, by decomposition, contribute to form one identical, invariable, essentially nutritive principle, - the "quad nutrit" of ancient authors; whereas, nothing is more clear, than that the nature and composition of the chyle will vary with each individual aliment.

89. Having explained all that is known with respect to the formation of chyle, we may next consider the manner in which it is absorbed and carried into the blood. The chyliferous vessels by which this office is performed have been already described (59). It is probable that the mesenteric glands, through which they pass, produce an important change on the chyle, but the nature of this change is wholly unknown: it is certain that these glands receive many blood-vessels, in proportion to their volume, and that they secrete a peculiar fluid, which may be extracted by compressing them between the fingers: whence some physiologists have supposed, that they add a fluid to the chyle in order to purify it; while others, again, have contended, that their use is to produce a more intimate mixture of the elements which compose it. I have already hinted my belief that something, partaking more of an organizing process, is effected by this glandular structure. Much discussion has also arisen upon the existence of the tact, or sensibility of these vessels; and although M. Majendie ridicules the supposition, there are not wanting facts to support the belief, that their mouths, like the pylorus, possess the power of discriminating between chyle and other less congenial fluids, which enables them to absorb the former and to reject the latter: and it is equally probable, that this selecting tact may be destroyed by disease, from which many evil consequences may arise.

The chyle is poured into the thoracic duct, together with the lymph which is brought hither from every part of the body by the lymphatic vessels, and thence carried into the subclavian vein, to be submitted to the action of the respiratory organs. It will, therefore, be remembered, that a portion of the decayed and broken down materials is conveyed into the lungs, together with the new materials which are to repair the waste.

90. The nutritive principles of the aliment have now been traced, through all their changes, to the circulation. Let us then return to the excrementitious part which was left in the duodenum. This matter is pressed forward, by the peristaltic motion of the intestine, losing as it proceeds any portion of chyle which may have escaped the lacteals in the higher part of the small intestines, into the caecum: its return being prevented by the valve already described (22).

91. Since the last edition of this work, Dr. Schultz of Berlin has very ingeniously maintained that where the food is of a vegetable nature, it undergoes a second digestion in the ccecum. He argues that in vegetable eaters, a large quantity of digestible, but as yet undigested matter, passes with the chyme into the intestine, and would be lost as refuse, if another digestion did not take place in the caecum, whereby the residual part of the nourishing matter is separated from the food. Now what proofs have we of the existence of such a function? In the first place, there is ample evidence to show that vegetable substances undergo some important changes in the intestinal canal beyond those which take place in the duodenum. The Baron Dupuytren in vigilantly observing the evacuations of those who had intestinal openings, at different distances from the stomach, invariably found that the ingesta always presented themselves at the opening of the wound in the inverse order of their digestibility; for example, fresh vegetables, still retaining their characteristic structure, appeared first, whereas animal matter was scarcely to be recognised. In examining the former, however, after its natural evacuation through the rectum, the vegetable structure, above described, had wholly disappeared.

If we apply to the comparative anatomist for assistance on this occasion, he will point out to us the digestion of the horse, in which it is evident that the intestinal canal performs an essential part; he will also compare the developement of the caeciim in herbivorous and carnivorous animals, and show that in the latter the stomach is nearly the only organ of activity. What further evidence is there in proof of the caecum being the seat of this second digestion? Viridet had remarked that in rabbits the food became a second time sour, after it had been neutralized in the duodenum; and this fact has been since established by repeated experiments; while in its future progress it is once more neutralized by the access of bile; changes which correspond exactly with those the food undergoes in the stomach and first intestine. Now it is evident that, in consequence of this twofold consumption of bile in the duodenum and caecum, there must exist an "antagonism" between the two digestions; for when the bile is consumed by the digestion in the duodenum, the caecal digestion cannot be pelfected; and on the other hand, when the bile flows into the caecum the neutralization of the acidity in the duodenum cannot take place.

It is probable that in those cases in which the caecal digestion is most perfectly developed, this antagonism is so prevented that each digestion has its particular period of action, so that when the one is in progress, the other is either lessened, or at rest. If these views be correct, we have no longer any difficulty in assigning to the valve of the caecum its proper office; for, that caecal digestion may take place, the still indigestible mass must be acidified, as in the stomach; now this could never happen, if the bile was allowed to flow without interruption into the caecum; its opening into the small intestine is therefore closed during the process, as the stomach is closed during chymification, with such a difference only as the different state of circumstances requires; thus, the stomach is closed at its pyloric orifice to prevent the egress of food, and the caecum at its iliac opening to prevent the ingress of bile.