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ERRATA AND OMISSIONS IN THE REFERENCES TO THE FIGURES
IN VOL. I.
Fig. 17, page 114, for “os orbiculare between the malleus and stapes,” read
“ between the incus and stapes.”
2. Lesser cornu of do.
1, 1. Thyroid cartilage.
6. Cricoid cartilage.
HUMAN PHYSIOLOGY. .
In the consideration of the preceding functions we have seen the alimentary matter subjected to various actions and alterations; and, at length, in the small intestine, possessed of the necessary physical constitution for the chyle to be separated from it. Into the mode in which this separation,—which we shall find is not simply a secerning action, but one of elaboration and of a vital character,—is effected, we have now to inquire. It belongs to the function of absorption, and its object is to convey the nutritive Auid, formed from the food, into the torrent of the circulation.
Absorption is not, however, confined to the formation of this fluid. Liquids can pass into the blood directly through the coats of the containing vessel, without having been subjected to any elaboration; and the different constituents of the organs are constantly subjected to the absorbing action of vessels, by which their decomposition is effected, and their elements are conveyed into the blood; whilst antagonizing vessels, called exhalants, deposit fresh particles, in the place of those that are removed. Yet these various substances,—bone, muscle, hair, nail, as the case may be, are never found, in their compound state, in the blood ; and the inference, consequently, is, that at the very mouths of these absorbents and exhalants, the substance, on which absorption or exhalation has to be effected, is reduced to its primary constituents, and this by an action, to which we know nothing similar in physics or chymistry: hence, it has been inferred, the operation is one of the acts of vitality.
All the various absorptions may be classed under two heads:the external and the internal; the former including those, that take place on extraneous matters from the surface of the body or from its prolongation--the mucous membranes; and the latter, those that are effected internally, on matters proceeding from the body itself, by removing parts already deposited.
By some physiologists, the action of the air in respiration has been referred to the former of these; and the whole function of absorption has been defined; the aggregate of actions, by which nutritive substances—external and internal—are converted into fluids, which serve as the basis of arterial blood.
The function of respiration will be investigated separately. Our attention will, at present, be directed to the other varieties, and, first of all, to that which occurs in the digestive tube.
Sect. I.-DIGESTIVE ABSORPTION.
The absorption, effected in the organs of digestion, is of two kinds; according as it concerns liquids of a certain degree of tenuity, or solid food. The former, it has been remarked, are subjected to no digestive action, but disappear chiefly from the stomach, and the remainder from the small intestine; whilst the latter undergo conversion, before they are fitted to be taken up from the intestinal canal.
Anatomy of the Chyliferous Apparatus. In the lower animals, absorption is effected over the whole surface of the body, both as regards the materials, necessary for the nutrition of the body, and the supply of air. No distinct organs for the performance of these functions are perceptible. In the upper classes of animals, however, we find an apparatus, manifestly intended for the absorption of chyle, and constituting a vascular communication between the small intestine and the left subclavian. Along this channel, the chyle passes, to be emptied into that venous trunk.
The chyliferous apparatus consists of the chyliferous vessels, mesenteric glands and thoracic duct. The chyliferous vessels or lacteals, arise from the inner surface of the small intestine; in the villi, which are at the surface of, and between, the valvulæ conniventes. Their origin is, however, imperceptible, even by the aid of the microscope; and, accordingly, the nature of their arrangement has given occasion to much diversity of sentiment, amongst anatomists. Lieberkuhn affirms that, by the microscope, it may be shown, that each villus terminates in an ampullula or oval vesicle, which has its apex perforated by lateral orifices, through which the chyle enters. The doctrine of open mouths has been embraced by Hewson, Sheldon, Cruikshank, and by many of the anatomists of the present day; but, on the other hand, it has been contested by Mascagni and others; whilst Rudolphi and Meckel believe, that the lacteals have not free orifices in the cavity of the intestine ; but that in the villi, in which absorption is effected, a spongy or sort of gelatinous tissue exists which accomplishes absorption, and, being continuous with the chyliferous vessels, conveys the product of absorption into them.
All these are mere speculations, too often entirely gratuitous;