The diameter of the thoracic duct varies, as we have seen, in different parts of its course. At the receptacle of the (byte it is sometimes nearly a third of au inch in diameter; and where it terminates in the veins, its dia• meter is commonly a or of an inch; while in its mid dle, and most contracted part, it is scarcely more than half a line. It is generally described as about as large as a crow quill. It runs in a waving direction.
We have, in compliance with custom, described the thoracic duet as single ; but it very commonly happens that there are two ducts, one on the lelt and the other on the right side, though that on the right side is general ly cxtremly short. In a few cases, the thoracic duct is double through its w hole length, and each of its divi sions terminates in separate parts of the sys tem; one in the usual angle on the left side, the other in the corresponding angle on the right.
We have said that the lacteals are filled with chyle, and the lymphatics with a transparent watery fluid, which is called lyinph. We must briefly notice the general appearance and chemical properties of these fluids.
The lymph is a wotery liquor, usually transparent, though, when subjected to sufficient heat, it is coagula ble. It can seldom be obtained in any quantity from the lymphatics, eacept w hen these are accidentally divided in the living body, and in these cases it sometimes distils from the wound, so as to impede its healing. The lymph etaitained in the lymphatics has scarcely been examined by chemical analysis, but so far as can be ascertained, it is of the same nature with that which is collected within the cavities of the body, and which is very simi lar to the scrum of the blood.
The chyle contained in the lacteals is a fluid of great er importance ; but unfortunately our acquaintance with this fluid is not very extensive. In its general appear ance it resembles milk, being of a white colour, coagu lable by heat, containing a fatty matter resembling cream, a sweetish substance like sugar, and a lew neutral salts, the nature or which has not been ascertained. Sec CHEM ISTRY.
Several morbid appearances are frequently discovered on dissection, in the absorbent system. The lymphatic vessels are often inflamed, and in this state are distinctly seen below the skin like red lines, which when touched, feel like hard tense cords, and are very painlnl. The lymphatic glands, especially those of the neck and groin, are frequently inflamed, and very readily pass into a state of suppuration, as in scrofula and syphilis. The lymphatic glands are sometimes found in a scirrhous state. This more especially happens to the glands of
the arm-pit, in cases of cancerous breast. They have also been seen bony. The mesenteric glands are fre quently diseased. Sometimes they are obstructed, and in these cases their size is greatly increased; they appear like boiled yolks of eggs, and on being cut open, are found to contain a whitish or yellowish curdy matter. They arc sometimes cancerous, when any part of the intestines in their neighbourhood is affected by that dis ease ; and in a few instances they have been found filled with an earthy or bony matter. The thoracic duct is sometimes grt atly enlarged, a remarkable instance of which enlargement is described and ligured by Mr Cruikshank. Sometimes it is obstructed by an earthy matter deposited within its cavity, and in a very few in stances it has been seen ruptured.
In no part of the animal structure have the investiga ions of the later anatomists been more successful than in the absorbent system. Indeed this system may be considered as having been entirely unknown belitre the year 16'27, when Asellius published his account of the lacteals, which he had first observed in 1622. It was only in the interior animals, how ever, that Asellius saw the lacteal vessels; and the first person who appears to have been favoured with a sight of them in man, was Vcslingius, in 1634. This anatomist appears also to have been the first who saw the w ac cording to Haller, was discovered by Veslingius in 1649, though no account of it was published till that of Pee quet, in 165I. Much about this time, the Swedish ana tomist Rudbec, discovered the lymphatic vessels, an ac count of which was first published in 1653, by Bartholin. It appears that the lymphatics had been seen in Eng land by Dr Jolyffe, so early as 1653, as Glisson, in his work De T'rntriculo et Inteslinis, published in 1654, in forms us ; but Jolyffe seems not to have understood their nature or uses. The valves of the absorbent vessels were first seen by Swammerdain in 1664, and they were de scribed by Ruysch in the following year. Both Swammer dam and Ruysch understood the method the absorbent vessels by injection ; but this important art was greatly improved by Nuck, who, in 1691, explain ed his method of injecting the absorbents with quick silver. This method was farther improved by Sheldon, and has been practised with great success by the disci ples of Dr William Hunter in London, the second Dr Munro in Edinburgh, and by Mascagni in Italy.