ORGANS OP ABSORPTIOC The absorbents are a minute kind of vessels ftiund in animal bodies, which at tract and imbibe any fluid that is brought near their. mouths. They are so minute and trrsparent, as not to be discovered in ordinary -section ; but by great la bour they have at length been detested in great numbers in every tribe of animals. As these vessel" are transparent, their contents are visible, which circumstance occasioned them to receive the different denominations of lacteals and lamphatics. The former svere so called, because they imbibed the chyle, a milky fluid, from the whilst the latter, containing much lymph, which they had taken up from all the interstices of the body,were therefore named lymphatics. The discovery ofthis system of vessels is referred to the seven teenth century. But at first their number did not appear sufficient to perform the whole function of absorption ; neither had they been discovered in birds or fishes, whence anatomists still retained the idea that the veins participated in this impor tant office. The merit of first demonstra ting the absorbing vessels in those animals belongs to Mr Howson, who assisted in the labours of the first eminent anatomical school in London, where anatomy was most ably taught by Dr. Hunter. And it is to the immortal Hunter that we are in debted for fully proving the important doctrine, that the whole business of ab sorption is performed by-the vessels which we arc now considering. They have of late been injected in such great abun dance, that they appear fully adequate to perform their office.
If, as we firmly believe, these vessels be the only ones which perform the office of absorption, they must exist in every part of the body. For there is no spot on the surface of the skin from which ointment may not be taken up, nor any internal part from which blood, when accidentally effused, may not be absorbed; nay, the very matter composingthe texture of our bodies is undergoing continual removal and renovation. These vessels must there fore be supposed to begin by open orifices generally throughout the body, although the fact can be demonstrated in the intes tines only. On the inner surfaces of these organs they appear to the unaided eye tine and pointed tubes : but by the mi croscope theirmouths are discerned to he pandous, and like a cup. The beginning absorbents soon join together, and after some time form minute vessels, capable of being injected by anatomists : these again conjoin, and form larger vessels, which are still discoverable with great diffi culty.
In structure and arrangements these ves sels have great similitude to veins : they have in ccmsequence been namedby some anatomists the lymphatic veins. like the veins, their sides are thin and transparent, though of considerable strength : like the veins, they frequently communicate toge ther, or, ash is technically termed, anas tomose. The advantage derived from these communications is obvious : for by these means, the dissimilar matters which they take up from various parts are mix ed together, and blended with the lymph, which they imbibe from the intersticesof the body, and which serves as the vehi cle for such heterogeneous particles ; they also prevent accidental pressure made on a few vessels from obstructing the progress of the absorbed fluids, which are in that case conveyed forwards by collateral channels. Like the veins also, these tubes, by conjoining, formn tnhe of smaller area than the united areas of the vessels before their junction. The effect of this construction is the same as in the veins ; that is, an acceleration in the cur rent of the lymph, in proportion as it conies nearer to the trunk of the absorb ing vessels. The diameter eFthe thoracic duct bears but a small proportion to the united diameters of all the minute ab sorbents in the body, and when this duct has been opened, the lymph has flowed from it with a force and jet like that with which the blood issues from a large vein. Like the veins, the absorbents are furnish ed with numerous valves, which prevent any retrograde motion of their fluids, and also prevent any portion of the vessel from sustaining tke weight of more fluid than is contained between its valves. The absorbents, however, difFerfrom the veins in one very material circumstance, viz. that they have a power of contraction, and are able of themselves to propel their contents. Whoever reflects on the phe nomena of absorption can scarcely doubt that these vessels have a contractile pow er, by which they refuse admission to noxious substances, whilst they readily imbibe those that are salutary. If these vessels are observed in the mesentery, when-turgid with absorbed chyle, their contents will disappear in a certain tract, and again become visible ; a phenomenon that can only be explained by supposing the vessel to contract at that part, and urge forwards its contents. Haller found that the thoracic duct contracted when stimulated, so that there can be little doubt of these vessels being muscular throughout their whole extent.