cells and cell-processes into flattened cellular elements is usually a subsequent process.

The mode of extension of the vascular system in growing parts of older animals, as well as in morbid new formations, is quite similar to that here described, except that blood-corpuscles are not developed within the cells which are forming the blood-vessels.

The blood-vessels may be said to increase in size and capacity in proportion to the demands made on their service. Thus, as the uterus enlarges in pregnancy, its vessels become enlarged, and when the main artery of a limb is tied, or otherwise permanently obstructed, collateral branches, originally small and insignificant, augment greatly in size, to afford passage to the increased share of blood which they are required to transmit, and by this adap. tation of them to the exigency, the circulation is restored. In such cases, an increase takes place in length, as well as in diameter, and accordingly the vessels very commonly become

tortuous.

RECENT LITERATURE OF BLOOD-VESSELS.

Epstein, S., Ueber die Structur normaler und ektalischer Venen, Virchow's Archiv, Bd. 108,

1887.1

Flemming, W., Ueber die Theilung der Pigmentzellen und Capillarwandzellen, Arch. f. mikr. Anat., xxxv., 1890.

Geberg, A., Ueber directe Anastomosen zwischen Arterien und Venen in der Nierenkapsel, Internat. Monatsschr. f. Anatom. u. Histolog., ii., 1885.

Jones, C. Handfield, Note concerning the endothelium of the small cerebral arteries, Journal of Anat. and Phys., vol. xxi., 1887.

Koeppe, H., Muskeln u. Klappen in den Wurzeln der Pfortader, Arch. f. Anat. u. Physiol., Physiol. Abth., Suppl. Band, 1890.

Kölliker, A. v., Histologische Studien an Batrachierlarven, Zeitschr. f. wissensch. Zoologie, Bd. xliii., 1886.

Mayer, S., Studien zur Histologie und Physiologie des Blutgefässsystems, Sitzungsber. der Wiener Akad., Bd. 93, iii. Abth., 1886.

Morpurgo, B., Ueber die Entwicklung der Arterienwand, Sitzungsber. d. Wiener Akademie, Bd. 90, iii. Abth., 1884.

Plotnikow, V., Untersuchungen über die Vasa vasorum, Dissert., Dorpat, 1884.

Relterer et Robin, Ch., Sur la distribution des fibres élastiques dans les parois artérielles et veineuses, Journal de l'anat., etc., 1884.

Spalteholz, W., Die Vertheilung der Blutgefässe im Muskel, Abhandl. d. math.-phys. Kl. d. Sächs. Gesellsch. d. Wissensch., Bd. xiv., 1888.

Westphalen, W., Histologische Untersuchungen über den Bau einiger Arterien, Dissert., Dorpat, 1886; Ueber die Intima der Arteria uterina, Virchow's Archiv, Bd. 106, 1886.

1 Epstein's paper may especially be consulted for many details regarding the differences of structure of different veins, as well as for references to the older literature of this subject.

LYMPHATIC SYSTEM.

Under this head are included not only the vessels specially called lymphatics or absorbents, together with the glands belonging to them, but also those named lacteal or chyliferous, which form part of the same system, and differ in no respect from the lymphatics, save that they not only carry lymph like those vessels, but are also employed to take up the chyle from the intestines during the process of digestion and convey it into the blood. The serous and synovial membranes may also be conveniently described along with the lymphatic system, since they are—especially the serous membranes-in close relationship to the lymphatics.

A system of lymphatic vessels is superadded to the sanguiferous in all classes of vertebrated animals, but this is not the case in the invertebrata; in many of these, the sanguiferous vessels convey a colourless or nearly colourless blood, but no additional class of vessels is provided for conveying lymph or chyle.

Distribution. In man and those animals in which they are present, the lymphatic vessels are found in nearly all the textures and organs which receive blood; the exceptions are few, and with the progress of discovery may yet possibly disappear. It is, however, with the connective tissue of the several textures and organs that the lymphatics are most intimately associated; indeed, as we shall immediately have occasion to notice, these vessels may be said to take origin in spaces in that tissue. The larger lymphatic trunks usually accompany the deeplyseated blood-vessels; they convey the lymph from the plexuses or sinuses of origin towards the thoracic duct. The principal lymphatic vessels of a part exceed the veins in number but fall short of them in size; they also anastomose or intercommunicate with each other much more frequently than the veins alongside of which they run.

It not unfrequently happens that a lymphatic vessel or a close interlacement of lymphatic vessels, may ensheath an artery or vein either partially or wholly. In this case the lymphatic is termed "perivascular."

Origin. Two modes of origin of lymphatic vessels are described, viz., the plexiform and the lacunar or interstitial, but no sharp line of distinction can be drawn between them, the difference depending chiefly upon the nature of the tissue or organ to which the lymphatics are distributed. Thus in flat, membranous or expanded parts, the lymphatic vessels usually form a network which is situated either in a single plane, as in many parts of the serous membranes, or in two or more planes united by intervening vessels, as in the skin and some mucous membranes. In the latter case the strata are generally composed of finer vessels, and form a closer network the nearer they are to the surface of the membrane in which they are distributed, but even the most superficial and finest network is composed of vessels which are larger than the sanguiferous capillaries.

The lymphatics of origin are often very irregular in size and shape (fig. 436, b, c). In them the lymph is collected, and it is conveyed away from the tissues and organs by more regular vessels provided with valves (fig. 436, a), which again combine to form larger lymphatic trunks.

Here and there vessels are seen joining the plexuses of origin which arise in the tissue by a blind and often irregular extremity. A long-known and well-marked example of such a mode of commencement is to be found in the lacteals of the intestinal villi, which, although they form networks in the larger and broader villi, arise in others by a single vessel beginning with a blind or closed extremity at the free end of the villus, whence it sinks down to join the general plexus of the intestinal membrane.

On the other hand in the more solid organs the lymphatic vessels occupy the interstices of the organ, and in many cases lose in great measure their character of distinct tubular canals, and appear simply as cleft-like spaces; these are, however, always bounded by an endothelial layer, like that which lines the lymphatic vessels elsewhere.

The lacunar mode of origin of lymphatics was first described in the testicle by

Fig. 436.-LYMPHATIC PLEXUS OF CENTRAL TENDON OF DIAPHRAGM OF RABBIT, PLEURAL SIDE.

a, larger vessels with lanceolate cells and numerous valves; b, c, lymphatics of origin, with wavybordered cells. Here and there an isolated patch of similar cells.

Ludwig and Tomsa, and it is now known to be characteristic of most glandular organs. Occupying everywhere the interstices of the penetrating connective tissue, the lymph bathes the exterior of the tubules or alveoli of the gland, in many parts even separating them from the capillary blood-vessels, so that the exchanges of material between the plasma of the blood and the secreting cells of the gland must be carried on through the intermedium of the lymph in these spaces. A network of

lymphatic spaces is also met with between the anastomosing muscular fibres of the heart.

What may be regarded as a third mode of origin of lymphatics is to be found in the open communications which subsist between the serous cavities and the lymphatic vessels in their walls. These orifices or stomata, which will be described with the serous membranes, allow of the passage of lymph from the serous cavities into the lymphatics, so that those cavities may, in a certain sense, be looked upon as large lymph lacunæ. Owing to this communication fluid is not, under normal circumstances, suffered to accumulate in them.

In some of the lower animals the lacunar condition of lymphatics has been long known. Rusconi found that the aorta and mesenteric arteries of amphibia are inclosed in large lymphatic spaces. Johannes Müller recognised the spaces which so extensively separate the frog's skin from the subjacent muscles as belonging to the lymphatic system, and Recklinghauser showed that the subcutaneous lymph-spaces of the frog's leg communicate with lymphatic vessels which envelope the blood-vessels of the foot; also that milk injected into these spaces finds its way into the blood. The lymphatic system, in being thus constituted by lacunæ or interstitial receptacles, so far agrees with the sanguiferous system of crustaceans and insects.

Structure. In structure the larger lymphatic vessels much resemble the veins, except that their coats are thinner, so thin and transparent indeed that the contained fluid can be readily seen through them. When lymphatics have passed out from the commencing plexuses and lacunæ, they are found to have three coats. The internal coat is covered with an epithelial lining (endothelium), consisting of a single layer of flattened nucleated cells, which have mostly an oblong or lanceolate figure, with an indented or bluntly serrated border, by which the adjacent cells fit to each other (fig. 436, a). Outside the endothelial layer the inner coat is formed of a layer or layers of longitudinal elastic fibres. The middle coat consists of plain muscular tissue disposed circularly, mixed with finely reticulating elastic fibres taking the same direction. Over the dilatations which occur in the vessels beyond each of the valves, the circular disposition of the muscular fibres gives place to a more irregular disposition, taking the form of an intricate interlacement of fibres. The external coat is composed mainly of white connective tissue with a sparing intermixture of longitudinal elastic fibres, and some longitudinal and oblique bundles of plain muscular tissue. In the thoracic duct there is a sub-epithelial layer (as in the arteries); and in the middle coat there is a longitudinal layer of white connective tissue with elastic fibres, immediately within the muscular layer. The muscular fibres of the middle coat, although for the most part transverse in direction, are nevertheless many of them oblique or even longitudinal.

The largest lymphatics have blood-vessels ramifying in their outer coat.

The commencing lymphatics or lymphatic capillaries, whether in plexuses or single (as in the villi), have a much simpler structure, their wall being entirely formed of a layer of endothelium either similar in form to those lining the larger vessels or (more frequently) presenting a characteristic waved border like the epidermic cells of grasses and some other plants (fig. 437).

Gaskell has described an attachment of elastic fibres to the walls of smaller lymphatics in some parts, and infers that the patency of the lumen of these vessels may by this means be restored after it has been temporarily obliterated by pressure (or by contraction of the muscular coat).

Valves. The lymphatic and lacteal vessels are furnished with valves serving the same office as those of the veins, and for the most part constructed after the same fashion. They generally consist of two semilunar folds arranged in the same way as in the valves of veins already described, but deviations from the usual structure here

RELATION OF LYMPHATICS AND CONNECTIVE TISSUE.

379 and there occur. A difference is found in the epithelium upon the two surfaces of the valves similar to that which has been noticed in the valves of the veins.

Valves are not present in all lymphatics, but where they exist they follow one another at much shorter intervals than those of the veins, and give to the lymphatics when distended, a beaded or jointed appearance. Valves are placed at the entrance

The lymphatics of fishes and amphibia are, generally speaking, destitute of valves, and may therefore be injected from the trunks; and valves are much less numerous in the lymphatics and lacteals of reptiles and birds than in those of mammiferous animals.

Relation of the lymphatics of origin to the cells and cell-spaces of the connective tissue. It has been already stated (p. 233) that the cells of the connective tissue lie in spaces in the ground-substance which they more or less completely fill. These cells and cell-spaces form in many parts an intercommunicating network of varying fineness extending throughout the substance of the tissue (fig. 437, c, fig. 438, d, d, fig. 439, c, c), whilst in other parts the cells acquire a broad flattened form, and joining edge to edge with other similar cells may in this way form an epithelioid patch in the ground substance. Not unfrequently the cells in such a patch take on the wavy border described above as met with in the lymphatics of origin (see the isolated patches in fig. 436). Further, the flattened cells which form the walls of the lymphatics are connected here and there both with the more ramified cells of the tissue (fig. 438, e) and with those which form the epithelioid patches, and in silvered preparations they appear to be continuous with one another. The epithelioid patches look in fact like a part of the lymphatic vessels, and are often regarded as such; it must be understood, however, that the spaces here spoken of, whether containing single cells or groups, are not true vessels, but merely vacuities in the ground-substance of the tissue containing flattened cells, which do not form a continuous vascular wall. And although the spaces present a very close relation to the lymphatic vessels, they can hardly be considered as actually opening into them by patent orifices, for the lymphatics proper have a complete wall of flattened cells united by a small amount of intercellular substance: at the same time this thin film can offer but a very slight resistance to the passage of fluid from the tissue into the vessel, or even to the passage of leucocytes or migrating cells, which, as is well known, penetrate the at least equally closed wall of the blood-vessels.

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