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into that bowel, and is there mixed with the aliment, as soon almost as it passes the stomach: adding also as a remark, how grievoufly this fame bileoffendsthe stomach itself, yetcherishes the vessel that lies next to it.
Secondly, We have now the aliment in the intestines, converted into pulp, and, though lately consisting of perhaps ten different viands, reduced to nearly an uniform substance, and to a state fitted for yielding its essence, which is called chyle, but which is milk, or more nearly resembling milk than any other liquor with which it can be compared. For the straining off of this fluid from the digested aliment in the course of its long progress through the body, myriads of capillary tubes, i. e. pipes as small as hairs, open their orifices into the cavity of every part of the intestines. These tubes, which are so fine and slender as not to be visible unless when distended with chyle, soon unite into larger branches. The pipes, formed by this union, terminate in glands, from which other pipes of a still larger diameter arising, carry the chyle, from all parts, into a common reservoir or receptacle. This receptacle is a bag large enough to hold about two table spoon-
W 2 fulls; fulls; and from this vessel a duct or main pipe proce ds, climbing up the back part of the chest, and then creeping along the gullet till it reach the neck. Here it meets the river. Here it discharges itself into a large vein, which soon conveys the chyle, now flowing along with the old blood, to the heart. This whole route can be exhibited to the eye. Nothing is left to be supplied by imagination or conjecture. Now, beside the subserviency of this whole structure to a manifest and necesfary purpose, we may remark two or three separate particulars in it, which shew, not only the contrivance, but the persection of it. We may remark, first, the length of the intestines, which, in the human subject, is fix times that of the body. Simply for a passage, these voluminous bowels, this prolixity of gut, seems in no wise necessary; but, in order to allow time and space for the successive extraction of the chyle from the digested aliment, namely, that the chyle, which escapes the latteals of one part of the guts, may be taken up by those of Rome other part, the length of the canal is of evident use and conduciveness. Secondly, we must also remark their peristaltic motion; which is madeupof contractions,following one
another another like waves upon the surface of a fl lid, and not unlike what we observe in the body of an earthworm crawling along the ground; and which is effected by the joint action of longitudinal and of spiral, or rather perhaps of a great number of separate semicircular fibres. This curious action pushes forward the grosser part of the aliment, at the fame time that the more subtile parts, which we call chyle, are, by a series of gentle compressions, squeezed into the narrow orifices of the lacteal veins. Thirdly, It was necesfary that these tubes, which we denominate lacteals, or their mouths at least, should be made as narrow as possible, in order to deny admission into the blood to any particle, which is of size enough to make a lodgement asterwards in the small arteries, and thereby to obstruct the circulation: and it was also necessary that this extreme tenuity should be compenfated by multitude; for a large quantity of chyle (in ordinary constitutions, not less, it has been computed, than two or three quarts in a day) is, by some means or other, to be passed through them. Accordingly, we find the number of the lacteals exceeding all powers of computation; and their
pipes so fine and slender, as not to be visible, unless filled, to the naked eye; and their orifices, which open into the intestines, so small, as not to be discernible even by the b. ft microscope. Fourthly, T he main pipe which carries the chyle from the reservoir to the blood, viz. the thoracic duct, being fixed in an almost upright position, and wanting that advantage of propulsion, which the an cries possess, is furnished with a succession of valves to check the ascending fluid, when once it has passed them, from falling back. These valves look upward, so as to leave the ascent free, but to prevent the return of the chyle, if, for want of sufficient force to push it on, its weight should at any time cause it to descend. Fifthly, The chyle enters the blood in an odd place, but perhaps the most commodious place possible, viz. at a large vein in the neck, so situated with respect to the circulation, as speedily to bring the mixture to the heart. And this seems to be a circumstance of great moment; for had the chyle entered the blood at an artery, or at a distant vein, the fluid, composed of the old and the new materials, must have performed a considerable part of the circulation, before it received that churn
ing in the lungs, which is, probably, necessary for the intimate and perfect union of the old blood with the recent chyle. Who could have dreamt of a communication between the cavity of the intestines and the left great vein of the neck P Who could have suspected that this communication should be the medium through which all nourishment is derived to the body? Or this the place, where, by a side inlet, the important junction is formed between the blood and the material which feeds it?
We postponed the consideration of digestion, lest it should interrupt us in tracing the course of the food to the blood; but, in treating of the alimentary system, so principal a part of the process cannot be omitted.
Of the gastric juice, the immediate agent, by which that change which food undergoes in our stomachs is effected, we shall take our account, from the numerous, careful, and varied experiments, of the Abbe Spallanzani.
1. It is not a simple diluent, but a real solvent. A quarter of an ounce of beef had scarce touched the stomach of a crow, when the solution began.
2. It has not the nature of faliva: it has not the nature of bile; but is distinct from both.