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glands, ultimately join the mesenteric lacteals, and the contents or the whole intestinal system of absorbents are carried by a few trunks to a chyle-receptacle,' fig. 399, 11, at the root of the mesentery, whence are continued the beginnings of the 'thoracic duct.' §342. Lymphatics.-These differ from the lacteals only in the nature of their contents, and even this is a temporary or contingent difference, for the lacteals convey a clear lymph, when the function of chylification is suspended. The gastric absorbents accompanying the right gastro-epiploic vessels communicate behind the beginning of the duodenum with lacteals' and absorbents from the liver the gastric absorbents from the lesser curvature join those of the liver descending Glisson's capsule:' the absorbents accompanying the left gastro-epiploic vessels unite with those from the spleen. The pancreatic absorbents communicate partly with the splenic ones, partly with the duodenal lacteals. The deep-seated absorbents of the liver, continued from the initial plexuses already adverted to in the portal fissures, fig. 373, emerge with the hepatic ducts, and are joined by those of the gall-bladder and by many of the superficial absorbents: they traverse glands in Glisson's capsule.' Some of the superficial absorbents ascend along the coronary and lateral ligaments' and enter the thorax, independently of the trunks of the deeper-seated They combine with the absorbents of the heart and lungs and those accompanying the internal mammary vessels to form three or four trunks communicating with the thoracic duct. The direct work of taking up waste tissues is done by independent organites: the earliest recognition of absorbents is as intercellular spaces or areola (vol. i. p. 455), or serous cavities; the canals continued from which, when filled by injected fluid, resemble a plexus,' such as Breschet has delineated in figure 393: such plexiform beginnings are commonly superficial, as beneath the skin and the serous surface of organs: in the substance of organs and tissues the origins are lacunar': in both forms the free surface shows aucleate scale-cells. When a distinct wall can be defined, the lymphatics of Mammals are seen to be more numerous, minute, and highly finished' than in lower Vertebrates. And, though remarkable for their almost transparent delicacy, their walls are strong, and in them may be distinguished fibrous layers

ones.

393

and a lining membrane: the latter consists of flat and nucleate epithelial cells, adherent to a reticulate subfibrous membraniform basis: it presents a smooth surface, like that of a serous membrane, to the naked eye. The fibres of the middle tunic affect a circular arrangement, are contractile like other fibres of the smooth system,' and are also elastic. An outer tunic may be defined by

a

2

396

the longitudinal course of the fibres of the condensed areolar tissue mainly forming it. In the thoracic duct longitudinal fibres of the smooth' kind are distinctly superadded to the outer coat, and a reticulate membrane has been detected between the inner and fibrous tunics. In the present class, the inner tunic is folded to form many and efficient valves, of the A 'semilunar' form, and commonly in pairs, fig. 394, rarely single: it is reflected from the fibrous coat half-way across the area of the vessel and then folds back upon itself to return to the wall, which it continues to line until it forms the next valve. The two layers of the fold firmly adhere, and offer great resistance to any pressure upon their concavity. In figure 395, a shows a side-view, b an oblique, and c an end-view of the usual disposition of the valves in pairs in distended lymphatics, when their free margins meet and Ca. close the area of the vessel to prevent the lymph flowing back. Mr. Lane has figured

B

phatics. CLXX".

three varieties in the valves of lymphatics, fig. Varieties of valves of Lym 396, near their entry into the conglobate bodies called glands.' In A, one fold, b, was less than the other b, and the margins of the outstretched folds did not meet or perfectly

close the vessel, the inner surface of which is shown at a: in B the folds were continuous forming a subcircular valve, and contained both fibrous and serous tissues: in c, besides the ordinary pair of semilunar valves, b, b, there was a subcircular fold, c.

§ 343. Absorbent ganglions.-These bodies, also called lymphatic or lacteal conglobate glands,' are much more numerous in Mammals than in other Vertebrates. In the limbs they are chiefly situated at the flexures of the joints; and, being connected by a looser tissue to surrounding parts, elude pressure by the freedom of motion so allowed. They occur in the neck and head external to the cranial cavity: in the thorax at the anterior and posterior mediastina,' and at the bronchial trunks where they are usually discoloured by black carbonaceous matter. In the abdomen they are found in the mesentery, near the spleen, and along the side of the aorta, post-caval, and iliac vessels. In the neighbourhood of the liver and gall-bladder post-mortem exudation tinges them yellow: as a rule, they are of a pinkish grey tint. The absorbents which enter the gland, fig. 397, в, a, a, are commonly smaller and more numerous than those that quit it, ib. e the former, or vasa inferentia' divide into small branches previous to entering. They then finely ramify, lose their proper tunics, and become continuous with those lacunar channels or ' vacuoles' which appear in the cell-mass of the developing glands. The preponderance of the fibrous tissue left, as it were, in the peripheral part of the gland gives ground for the distinction of a 'cortical' from a central' portion. But there is no definite boundary-line: septa extend from the cavernous' capsule, at first lamelliform in the cortical part and becoming cord-like or trabecular' in the central part. In the latter, the lymphchannels become larger, especially in the mesenteric glands, and have been termed 'loculi:' they are large in the mesenteric glands of the Cetacea, though not in the degree, or with the anatomical relations, described in CLXXIV", p. 27. They are paved by the flat nucleate cells, and usually contain a whitish pulpy matter: minute plexiform vessels, surrounding the loculi,' form the beginnings of most of the vasa efferentia,' ib. e; a few are direct continuations of the inferent vessels.

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§ 344. Disposition of Lymphatics.-In the Mammalian class the anatomical disposition of the lymphatic system has been most completely traced out in the human subject. Successfully injected, the superficial lymphatics of the lower limb present the general arrangement shown on the fore-part of CLXXVI". p. 152.

the leg, in Mascagni's magnificent work (CLXXI"), from which fig. 398 is reduced. On the inner side they tend to converge

about the vena saphena, and, with the deeper-seated ones, mainly unite into trunks which again subdivide to enter the inguinal glands,' fig. 399, 1, 2. Their efferent trunks affect the course of the iliac vessels, converging toward and uniting by cross branches with those of the opposite side, and communicating with the lacteal system, at the receptaculum chyli,' 11, whence proceed the origins of the thoracic duct. This, in Man and most Mammals, enters the thorax between the aorta and vena azygos, and lies behind the œsophagus in the posterior mediastinum. tuous and rarely single throughout. It often splits into two or more branches, which after a longer or shorter course reunite; this division and reunion may be two or three times repeated. The principal canal, in Man, fig. 400, a, a, mounts into the cervical region in front of the vertebral artery and vein to the level of the seventh cervical vertebra, opposite to which it begins to form a curve, first forward and outward, then downward

Superficial lymphatics of the lower extremity. CLXXI".

It is frequently tor

and inward, over the subclavian artery to reach the angle of union between the left subclavian, s, and internal jugular, j, veins, at which point it empties itself into the venous system by one or more branches. The corresponding veins on the right side also

Absorbent trunks of inguinal and lumbar regions, with receptaculum chyli; Human. CLXX.

receive lymph, but usually by a short trunk, ib. c. There have been observed, in Man, varieties which are more constant in some lower Mammals, as, e. g., a double thoracic duct,' one terminating in the left, the other in the right subclavian vein; a bifurcation of the duct at a higher or lower level, one branch terminating in the angle of union of the subclavian and internal jugular veins of the left side, the other emptying itself either into the corresponding point on the right side or joining the right lymphatic trunk,