The lymphatics are divided into deep-seated and su perficial, according as they arise from the central or peripheral pans of the body. The superficial lympha tics are the most numerous, and the most easily exa mined. They run immediately below the skin, and the glands through which they pass are very evident to the touch in several parts of the body, especially in the groins and arm-pits. The deep-seated lymphatics usual ly accompany the branches of the veins and arteries, and are supposed to be more than double their number. All these are nourished by very minute blood-vessels, and appear to be furnished with nervous fibres.
The lacteals that originate in the intestines, are ex tremely numerous, especially at their origin. They run beside the branches of the mesenteric arteries and veins, and form several considerable trunks, as they approach the trunks of these vessels The lymphatic glands are round or oval bodies, gene rally flattened, and of a reddish brown colour in young people, but grayish or yellowish in those of more ad vanced age. They are of very different sizes, some be ing not larger than a millet seed, while others are nearly an inch in diameter. They are found in various parts of the body, but are most remarkable in the arm-pits, the neck, the groins, the ancles, and about the joints of the knees. See Plate XX. They arc situated in the cellular substance, and are enveloped each in a membra nous covering, that is of a dense texture, and smooth appearance, and which seems to be derived from the cellular substance by which they are connected with the neighbouring parts. In their internal structure they are soft and pulpy, and seem to be composed of cells containing a whitish fluid. Some of them appear rather like a collection of minute vessels than of cells. They are all supplied with blond-vessels and nerves. The mesenteric glands do not differ essentially from the ()the' glands or the to,,,,,rbent ut, The) are generally of a consideralde size, of an o\ al from, See l'late XVIII. Fig. 12.) and of a whitish colour. They are situated be tween the folds of the, mesentery, and al e usually plat cd in groups of three or lieu' toy, thel) the large branches of the mesenteric vessels. All the lacteals do not pass through the mesenteric glands, several having been observed to creep over them.
The lymphatic glands have been compared to the ganglions of the nervous system, and are by some anato mists supposed to prodm e some important change on the fluids that pass through them. That the mesente ric glands are intended to prepare the chyle more com pletely before its entrance into the sangtiiferous system, there can be little doubt, though, from the circumstance mentioned above, it might appear that these glands were not absolutely necessary. As, however, nearly all the
lacteals pass through these glands, we must consider those instances in which they run over them in passing to the throracic duct, as only accidental exceptions in the general plan of Nature ; and as these vessels freely inosculate with each other, the fluid that each contains must be mixed and modified by that which passes through the others.
\Ve have said, that most of the absorbent vessels pass into lymphatic glands. \Ve may now remark, that similar vessels, though usually fewer in number, and of a larger diameter, pass out at the opposite sides of these glands. The entering vessels are called by ana tomists -vase infirentia, e. vessels carrying the absor bed matters into the glands,) and the emerging vessels vase efferentia, (or vessels carrying it out.) The circum stance of the emerging lymphatics being less numerous than the entering, proves that they are distinct vessels, and not, as some have supposed, a continuation of the same trunks passing through the glands.
The lacteals coming from the bowels, and the lym phatic vessels from all the sacral parts of the body, gra dually unite at about the the third vertebra of the loins, in a vessel of larger diameter than the largest branches of the lacteals or lymphatics; but of the same structure, both in its membranes and valves. This is the thoracic duct, which soon after its commencement swells into an oval cavity, of rather larger diameter than the rest of the tube, called the receptacle of the chyle. This re ceptacle is generally situated near the first vertebra of the loins, on the right side, a little higher than the re nal artery. From this part, the thoracic duct ascends between the crura of the diaphragm, into the chest, where it passes between the descending aorta and the a7.-ygos vein, growing gradually smaller till it reaches the mid dle of the back, where it again begins to dilate. At about the eighth dorsal vertebra, counting from above, it frequently divides into two tubes ; but these soon after reunite. Passing behind the aorta, it leaves the chest, and mounts upwards to the lower part of the neck, in clining towards the left side, behind the left subclavian vein. Having reached the neck a little above the sub clavian, it makes a turn downwards, and enters the ve nous system, at the angle formed by the junction of the left subclavian, with the left internal jugular vein. In its passage through the chest, it receives numerous absor bents from the viscera contained in that cavity, and from all the atlantal parts of the body, thus forming the gene ral centre of the whole absorbent system.