ORGANS of ABSORPTION.
THAT some kind of absorption takes place in ahnost That some kind of absorption takes place in ahnost every class of animals with which we are acquainted, there can be no doubt ; but an absorbent system is not found in several of the inferior classes.
The absorbent system of MANMALIA differs little, either in the structure or situation of its organs, from that of man. The lacteals and lymphatics have a simi lar structure, but in some species are proportionally larger and more tortuous than in man. The thoracic duct is also frequently larger in proportion to the body of the animal ; and that particular dilatation of it which we have described by the name of the receptacle of the chyle, is in general much more remarkable. Most of the lymphatic glands have much the same appearance as in man, but the structure of these, as well as of the me senteric glands, is, in many species, more evidently cel lular. In several animals of this class, especially in those of the orderfer.e, the mesenteric glands are collected into one conglomerate mass, commonly- called the pan creas of Asellius.
It has been lately discovered, that the general trunk of the absorbent system in the horse communicates freely with the neighbouring veins; and it is hence in ferred by a late writer on comparative anatomy, that the lymphatics generally communicate With the ve nous system prior to the termination of the thoracic duct. See Lawrence's Translation of Comp. Anat. p.
In BIRDS, the ltzctcals are not to be distinguished from the other lymphatic vessels, except in their si tuation, as in these animals the chyle is transparent. Their thoracic duct is always double ; but though lym phatic glands are found in several parts of the body in the larger birds, there are none attached to the mesen tery.
The absorbent system of REPTILES and SERPENTS is very simple, consisting omy of lymphatic vessels, and a double thoracic duct. The lacteals in this class are ex tremely numerous, and are spread over the mesentery, and over both the peripheral and central surfaces of the intestines, so as, when injected with mercury, to form a most beautiful appearance.
The absorbent system of FISHES resembles that of reptiles and serpents, in the want of glands, but here the vessels appear to be destitute of valves.
Lymphatic vessels have been described in the sea urchin, (echiires esculentus,) see Monro on Fishes, p. 88. and in the larva' oil NSECTS, see Sheldon on the Absorbent System, p. 28 ; but most of the later writers on compara tive anatomy are disposed not to consider these vessels as true absorbents.