The chief peculiarity of the coats of the lymphatic vessels is their remarkable thinness and transparency ; in other respects they bear considerable resemblance to the carts of the veins ; indeed in some of the lower animals the veins are nearly as thin, and when empty of blood, as transparent as the lymphatics. All anatomists admit the existence of two coats in the lymphatics, an internal serous lining, which at intervals is thrown into folds to form the valves, and an external thicker fibrous covering; to these is added by some anatomists, with a hom I am disposed to coincide, a third, analogous to the cellular tunic of the bloodvessels, which conveys to them their vasa vasorum, and by which they are connected to the surrounding structures.
The inner tunic is extremely fine and delicate, probably less elastic and extensible than is ge nerally imagined, and is the first to give way under distention from forced injections. It appears to possess a much closer texture than the fibrous tunic, to which it is firmly adherent, and whose contractions and dilatations it is compelled to follow. The epithelium scales are distinguished with difficulty on the inner surface of this tunic, but I have satisfied my self of their existence. On placing an opened lymphatic in the field of the microscope after the bloodvessels have been minutely injected, the vasa vasorum may be very distinctly seen ; but it is difficult, from the perfect trans parency. of this tunic, to say whether these vessels reach it, or are only seen through it. The vasa vasorum of the lymphatics do not appear to affect any constant or fixed arrange ment; they are by no means numerous, and I have never been able to detect any on the val vular folds.
The fibrous tunic, like the internal, is trans parent ; it is very elastic, and admits of consi derable distention without rupture. There is great variety of opinion with respect to the na ture of its tissue. Breschet and many other anatomists describe this tunic as resembling the cellular coat of the bloodvessels, and are under the impression that the lymphatics are I altogether deficient in that which is analogous to the middle or fibrous coat of the arteries and veins. Mascagni and Itudophi have not been able to detect muscular fibres in it. Cruveilhier conceives it to be composed of the tissu jaunt elastique, or tissu dartoide. Schreger thinks he has seen circular muscular fibres in the thoracic duct of man and of large animals, and Sheldon states that he has distinctly seen muscular fibres in the thoracic duct of the horse. By
placing a portion of the thoracic duct or large lymphatic laid open with the lining membrane uppermost, on a piece of glass, and by scra ping off the internal membrane, the fibrous tunic will be exposed ; if it be now moistened with a drop of water, and a piece of talc placed over it, it may be readily examined under the microscope. I have several times examined portions of the thoracic duct and of the larger lymphatics taken from the horse, and from the human subject, and have invariably found the tunic exposed on removing the lining mem brane, to be composed of fibres passing princi pally in the longitudinal direction ; these fibres are uniform and cylindrical, and resemble in these respects the organic muscular fibre as de scribed by Schwann ; they lie for the most part parallel with each other, and are occasionally seen to form a large flisciculus, somewhat analo gous to the longitudinal muscular bands of the large intestine. These fibres measure from 1-5000th to 1-6000th of an inch in diameter, and present at intervals, a sudden zigzag inflec tion; several fibres collected together into a sort of primitive fasciculus are bent together at the same points. These abrupt deviations from the straight line do not occur at equidistant points : the intervals between them differ greatly ; they average 1-400th of an inch in length. Under the lining membrane some few fibres may be distinguished taking a transverse course, others may be seen in an oblique direc tion, but the great majority are arranged longi tudinally. The primitive fibre of cellular tissue is freely mixed with the peculiar fibres just described. The physiological fact that the lymphatics have the power of contracting and emptying themselves of their contents, has not been disputed ; but with respect to the nature and form of the fibre in virtue of which they possess this faculty, there has been and still exists great uncertainty. No one can have ex amined the lacteal or lymphatic vessels in a recently killed animal without having observed the rapidity with which this system will empty itself of the fluid it contains ; and if the trunk of the system be ligatured, it will be found that this power remains for an hour or more after death, as may be proved by puncturing the duct within this period below the ligature, or by puncturing a distended lymphatic, which will be instantly evacuated of its contents, and will refill again and again when pressure is made below the orifice.