TEAL.
The anatomical structure of the lymphatics seems to be essentially similar to that of the lacteals ; they are composed of a firm elastic membranous substance, capable of consider able distention without being ruptured, and furnished with numerous valves; like the lac teals they form very frequent anastomoses. We have the same evidence of their contracti lity as of that of the lacteals, although we are perhaps still less able to demonstrate the actual existence of their muscular fibres. We presume that they are likewise analogous to the lacteals in the nature of their office, and in their desti nation, although they differ from them with respect to their situation, or the parts of the body to which they are attached ; the lacteals being confined to the membranes connected with the intestines, while the lymphatics are found in almost every part of the body, and connected with nearly all its various textures.* They differ also in the nature of the fluid which they contain, for while that of the lac teals, as has been stated above, is white and opaque, the fluid found in the lymphatics is transparent and colourless, so as to resemble water, from which they have derived their spe cific denomination.
It is very difficult, if not impossible, to trace the actual commencement of the lymphatics ; but partly from anatomical researches, and partly from physiological considerations, we are led to conclude that they originate from the various surfaces of the body, of all de scriptions, both internal and external. They resemble the lacteals, in passing from larger to smaller branches, which, after numerous anastomoses, unite in a few large trunks, the greatest part of which terminate in the thoracic duct. The great trunks of the lymphatics are, for the most part, arranged into two distinct series, one considerably more superficial than the other ; it is observed that they generally follow the course of the great veins, but it may be doubted whether any direct communication exists between them during their course, and we are not aware of any physiological cause of this arrangement.
With respect to the mouths or origin of the lymphatics there is even more uncertainty than with respect to that of the lacteals ; no anato mical investigation has hitherto been able to detect them, and although numerous facts of constant occurrence would seem to prove that their capillary extremities are distributed over all the surfaces of the body, it is from various pathological observations and from the analogy of the lacteals that we arrive at this conclu sion."
The thoracic duct is a vessel of considerable size, which is situated near the spine, and which extends from about the middle of the dorsal vertebrae to a short distance above the left subclavian vein ; here it assumes an arched form, and is bent down until it enters the vessel near its junction with the jugular vein of the same side.t The duct, in its passage along the spine, is deflected in various ways, and proceeds in a somewhat irregular or tortuous course. For the most part it consists of a single but occasionally there are two trunks, either of the same or of different sizes, and we have not unfrequently partial appen dages, which are added to the main trunk in different parts of its course. Besides what is properly considered as the thoracic duct, in which all the lacteals and the greatest part of the lymphatics terminate, a portion of these latter, especially those which proceed from the upper part of the body and from the superior extremity of the right side, are generally col lected into a separate trunk, named the great right lymphatic vessel, or right thoracic duct, which is connected with the right subclavian These irregularities in the disposition and form of the thoracic duct may be consi dered as inno respect affecting its physiological uses, and to be no more than an anatomical variation of structure, probably depending upon some mechanical cause. It is, however, a circumstance of considerable importance in respect to the pathological conclusions that have been sometimes drawn from the obstruc tions of this organ, as well as from the experi ments that have been performed upon it. The structure and properties of the thoracic duct appear to be similar to those of the large trunks of the lacteals and lymphatics ; its coats are comparatively thin and transparent, yet it is possessed of considerable strength, and is ca pable of being distended much beyond its ordinary bulk ; it is furnished with numerous valves, and exhibits a great degree of con tractility.