Fig. CXCVI.Fig. CXCVII.Fig. CXCVIII.
CXCVI.—1. Trunks of absorbent vessels entering a gland. 2. Gland laid open. 3. Highly magnified views of the cells or follicles of which the gland is supposed to consist. CXCVII.—1. Absorbent vessels called vasa inferentia, entering (2) the gland. 3. Absorbent vessels emerging from the gland, called vasa efferentia, and forming (4) a common trunk. CXCVIII.—1. Trunk of absorbent vessel entering a gland. 2. Gland apparently composed entirely of convoluted vessels. 3. Vessels emerging from the gland and forming (4) a common trunk.
830. In the human body every vessel that can be distinctly recognised either as a lacteal or a lymphatic, passes, in some part of its course, through a conglobate or lymphatic gland (figs. [CXCVII]., [CXCVIII].). These glands, small, flattened, circular or oval bodies, resembling beans in shape, are enclosed in a distinct membranous envelope. Their intimate structure has been already fully described (chap. xi.). They are of various sizes, ranging from three to ten lines in diameter: they are placed in determinate parts of the body, and are grouped together in various ways, being sometimes single, but more often collected in masses of considerable magnitude. Numerous absorbent vessels, termed vasa inferentia, enter the gland on the side remote from the heart (figs. [CXCVII]. 1 and [CXCVIII]. 1); a smaller number, called vasa efferentia, leave it on the side proximate to the heart (fig. [CXCVII]. 3). If mercury be injected into the vasa inferentia (fig. [CXCVI].), it is seen to pass into a series of cells of the corresponding gland (fig. [CXCVI]. 3), and then to escape by the vasa efferentia; but if the gland be more minutely injected, as by wax, all appearance of cells vanishes; the whole substance of the gland seems then to consist of convoluted absorbents (fig. [CXCVIII]. 2), irregularly dilated, and communicating with each other so intimately that every branch that leaves the gland appears to have been put in communication with every branch that entered it (fig. [CXCVIII]. 1, 2, 3).
831. The motion of the fluid within the absorbent vessels, though not rapid, is energetic. If a ligature be placed around the thoracic duct in a living animal, the tube will swell and ultimately burst, from the rupture of its coat, in consequence of the force of the distension that takes place below the ligature. If the thoracic duct in the neck of a dog be opened some hours after the animal has taken a full meal, the chyle flows from the vessel in a full stream, and in the space of five minutes half an ounce of the fluid may be obtained. Yet this system of vessels is beyond the influence of the circulating blood: it has no heart to propel it; no current behind always in rapid motion to urge it onwards; it is therefore inferred that it is moved by a vital contractile power inherent in the vessels, analogous to, if not identical with, muscular contractility. The flow of blood through the arterial tubes is universally believed to be effected, in part at least, by such a contractile power, for this, among other reasons, that if in a living animal the trunk of an artery be laid bare, the mere exposure of it to the atmospheric air causes it to contract to such a degree that its size becomes obviously and strikingly diminished (298.1). The same phenomenon has been observed in the main trunk of the absorbent system. Tiedemann and Gmelin state that in the course of their experiments they saw the thoracic duct contract from exposure to the air.
832. The delicacy and transparency of the lacteals and lymphatics long concealed them from the view of the anatomist. The lacteals had indeed been occasionally seen in ancient times, but their office was altogether unknown. In the year 1563 Eustachius discovered the thoracic duct, but did not perceive its use. About half a century afterwards, in the year 1622, the lacteals were again one day by chance seen by Asellius, in Italy, while investigating the function of certain nerves. Mistaking the lacteals for nerves, he at first paid no attention to them; but soon observing that they did not pursue the same course as the nerves, and “astonished at the novelty of the thing,” he hesitated for some time in silence. Resolving in his mind the doubts and controversies of anatomists, of which it chanced that he had been reading the very day before, in order to examine the matter further, “I took,” he says, “a sharp scalpel to cut one of these chords, but scarcely had I struck it when I found a liquor white as milk, or rather like cream, to leap out. At this sight I could not contain myself for joy; but turning to the by-standers, Alexander Tadinus and the senator Septalius, I cried out Εὕρηκα0! with Archimedes; and at the same time invited them to look at so rare and pleasing a spectacle; with the novelty of which they were much moved. But I was not long permitted to enjoy it, for the dog now expired, and, wonderful to tell, at the same instant the whole of that astonishing series and congeries of vessels, losing its brilliant whiteness, that fluid being gone, in our very hands, and almost before our eyes, so evanished and disappeared that hardly a vestige was left to my most diligent search.” The next day he procured another dog, but could not discover the smallest white vessel. “And now,” he continues, “I began to be downcast in my mind, thinking to myself that what had been observed in the first dog must be ranked among those rare things which, according to Galen, are sometimes seen in anatomy.” But at length recollecting that the dog had been opened “athirst and unfed,” he opened a third “after feeding him to satiety; and now everything was more manifest and brilliant than in the first case.” The zeal with which he followed out the clue he had obtained is indicated by the number of dogs, cats, iambs, hogs, and cows which he dissected, and by the statement that he even bought a horse and opened it alive; but, he adds, “a living man, however, which Erasistratus and Herophilus of old did not fear to anatomize, I confess I did not open.”
833. Nearly thirty years elapsed before the lacteals, which were long thought to terminate in the liver, were traced to the thoracic duct; and it was not until the year 1651, about eighty years after the discovery of Asellius, that the lymphatics were discovered, and that the whole of this portion of the absorbent system was brought to light.
834. Taking together the whole of the apparatus of absorption, the specific office performed by its several parts seems to be as follows:—
835. 1. It is established that the lacteals absorb chyle, and that they refuse to take up almost every other substance which can be presented to them. Experimentalists are uniform in stating that however various the substances introduced into the stomach, it is exceedingly rare to find in the lacteals anything but chyle. These vessels appear to be endowed with a peculiar sensibility, derived from the nervous system, by which they are rendered capable of exerting an elective power, readily absorbing some substances and absolutely rejecting others.
836. 2. The lymphatics absorb a far greater variety of substances than the lacteals, but not all substances indiscriminately; chiefly organized matter in a certain stage of purification; particles passing through successive processes of refinement ([707]).