ON ABSORPTION.
AFTER. having given an account of the means by whichAfter. having given an account of the means by which i fresh matter is provided, to repair the waste that is con tinually going forward in the system, it remains for us to i consider the function by which the particles that are n tended for the future supply of the body are conveyed into the blood, while, at the same time, those that have per formed their appropriate offices, and are therefore become useless or noxious, are removed from the body : this con stitutes the process of absorption. We shall first describe the apparatus by which this function is carried on ; 2. The use of the absorbent system ; and, in the third place, we shall offer some remarks upon the mode in which the ab sorbents are supposed to act.
The absorbent system consists or four parts; the lac teals, the lymphatics, the conglobate glands, and the thoracic duct. The lacteals are a system of vessels, the object of which is to take up the chyle from the intestines, and convey it into the circulation. They seem to have been imperfectly known to Galen, although he was mis taken as to their destination, which he supposed to be the liver. For some centuries they were entirely forgotten or disregarded, until they were again discovered, in the be ginning of the seventeenth century, by Asselli. They obtained the name of lacteals, in consequence of their contents having the consistence and appearance of cream. The lacteals are described as opening into the intestine by a number of small capillary tubes, termed villi, which radiate, as it were, from a centre, several of them uniting to form one vessel ; but there is still some uncertainty re specting the nature of these villi, and their connexion with the proper lacteals. The lacteals themselves are then carried along the mesentery. During their passage the small branches run together, and form larger and larger vessels, until at length they are all reduced to a few prin cipal trunks, which terminate in the thoracic duct. The lacteals are characterized by the thinness and transpa rency of their coats, and by the circumstance of their being furnished with numerous valves. From this struc ture it follows, that when they are distended, either by chyle or by injections, they assume a jointed appearance, something like a string of beads. During their course they frequently inosculate, and the inoseulations are Sometimes so numerous and intricate as to form a com plete plexus. They are supposed to possess a consider able degree of contractility, although, from their transpa rency and delicacy of texture, it is impossible to demon strate their fibres.
The lymphatics, both in their functions and their des tination, are analogous to the lacteals, except that whereas the latter are confined to the intestines, the former are dispersed over all parts of the body. They originate from every surface, external and internal, communicate with every organ, and at length terminate in the thoracic duct. In their structure they much resemble the lacteals, being, like them, possessed of muscular contractility, and pro vided with numerous valves ; but the fluid which they contain being, like themselves, nearly without colour, they are not easily detected ; and they were accordingly not discovered until some time after the lacteals. There has been a good deal of controversy respecting their dis covery ; but it is now generally agreed that the merit be longs jointly to Rudbeck and Bartholine, who, each of them, published their observations about the middle of the seventeenth ceniury, and seem to have pursued their investigations without any concert or communication. This department of anatomy has of late years been as siduously cultivated in this country, particularly by Hun ter and Monro, from whose observations we learn that the lymphatic system is distributed to every part of the body.
Both the lacteals and the lymphatics, in their course to the thoracic duct, meet with a number of glands, which have been termed, from their supposed structure, con globate ; but the relation which they bear to the vessels themselves is obscure ; nor indeed is their structure tho roughly understood, for it has been a much agitated ques tion, whether they contain proper cavities, or consist al together of a net-work of vessels. Upon the whole, the evidence appears to be in favour of the opinion, that the glands are entirely vascular, except the cellular sub stance necessary for connecting the vessels together. The glands are much more numerous in some parts of the body than in others ; but it is admitted as a general fact, that no lacteal or lymphatic arrives at the thoracic duct with out passing through one or more glands in its course. This remark, however, is only true so far as respects the human species, and those animals who possess a struc ture and organization the most nearly resembling him ; for these glands are said to be rare in birds, and in fish to be altogether wanting.