exposure that the white blood-corpuscles, which always tend to pass into the inert layer, and to adhere occasionally to the inner sur

face of the blood-vessels, here and there pass through the coats of the small vessels, and appear as migratory cells in the surrounding connective tissue.

LYMPHATIC SYSTEM.

To the lymphatic system belong not only the lymphatic vessels and lymphatic glands, but also the cavities of the serous membranes, which are moistened with lymph and are in open communication with the lymphatic vessels in their parietes.

The larger lymphatic vessels somewhat resemble the veins in structure, except that their coats are much thinner and their valves much more numerous. In lymphatics of somewhat smaller size, the wall of the vessel is formed, first, by a lining of pavement-epithelium cells (endothelium of some authors), which are elongated in the direction of the axis of the vessel; and, secondly, by a layer of circularly and obliquely disposed muscular fibres. In the smallest vessels (lymphatic capillaries), which, however, are generally considerably larger than the blood-capillaries, there is nothing but the epithelium remaining, and the cells of this are frequently not more elongated in one direction than in another, but have a characteristic wavy outline (fig. 121).

Lymphatics begin in two ways-either in the form of plexuses, as in membranes (fig. 120), or as lacunar interstices, as is the case in some of the viscera.

In order to show the lymphatic vessels, it is generally necessary to stain a tissue with nitrate of silver; but they may easily be injected by sticking the nozzle of an injecting canula into any tissue which contains them, and forcing coloured fluid under gentle pressure into the interstices of the tissue.

PLEURAL SIDE.

a, larger vessels with lanceolate cells and numerous valves; b, c, lymphatic capillaries with wavy-bordered cells.

In silvered preparations it may be observed that the lymphatics always appear in the form of clear channels in the stained ground-sub

FIG. 121.-A SMALL PART OF THE LYMPHATIC PLEXUS ON THE PLEURAL SURFACE OF THE DIAPHRAGM. (Magnified 110 diameters.) (Ranvier.)

L, lymphatic vessel with characteristic epithelium; c, cell-spaces of the connective tissue, here and there abutting against the lymphatic.

1, lymph-channel below the surface, lying between tendon bundles, 1, 1, and over which the surface-cells are seen to be relatively smaller, and to exhibit five stomata, s, s, leading into the lymphatic. The epithelium of the lymphatic channel is not represented.

H

stance of the connective tissue, and that their walls are in close connection with the cells and cell-spaces of that tissue. But, except in the case of the serous membranes, there is no open communication between the lymphatic vessels and the interstices (areola) of the connective tissue.

Development of the blood-vessels and lymphatics.-The bloodvessels and lymphatics are developed in the connective tissue or in the mesoblastic tissue which precedes it, the first vessels being formed in the vascular area which surrounds the early embryo. Both kinds of vessels are developed from cells (vaso-formative cells) which become hollowed out by an accumulation of fluid in their protoplasm, and in the case of developing blood-vessels coloured blood-corpuscles may also be formed within these cells (see Development of Blood-corpuscles, Lesson II.) The cells branch and unite with one another to form a network, and their cavities extend into the branches. In the meantime their nuclei multiply and become distributed along the branches, cell-areas being subsequently marked out around them. In this way intercommunicating vessels-capillaries containing blood or lymph— are produced (fig. 123). These presently become connected with

FIG. 123.-ISOLATED CAPILLARY

NETWORK FORMED BY THE JUNCTION OF SEVERAL HOLLOWED-OUT CELLS, AND CONTAINING COLOURED BLOOD-CORPUSCLES IN A CLEAR FLUID.

c, a hollow cell the cavity of which does not yet communicate with the network; p, p, pointed cell-processes, extending in different directions for union with neighbouring capillaries.

previously formed vessels, which extend themselves by sending out sprouts, at first solid, and afterwards hollowed out. It is not precisely known whether the larger blood-vessels and lymphatics are developed at first as capillaries, the muscular and other tissues being subsequently added, or whether they are formed as clefts in the mesoblastic tissue which become bounded by flattened cells.

The serous membranes, which may conveniently be studied in connection with the lymphatic system, are delicate membranes of connective tissue which surround and line the internal cavities of the body, and are reflected over many of the thoracic and abdominal viscera; in

passing to which they form folds, within which blood-vessels, lymphatics, and nerves pass to the viscera.

The inner surface is lined by a continuous layer of pavementepithelium (fig. 122), which is very distinct in nitrate of silver preparations. In some places there are apertures in the epithelium which lead direct into subjacent lymphatic vessels. These apertures are called stomata, and are surrounded by small protoplasmic cells (fig. 122, s, s). They are most numerous upon the peritoneal surface of the diaphragm, but are present in all serous membranes, and they serve to prevent any undue accumulation of lymph within the serous cavity during health. The pavement-epithelium rests upon a homogeneous basement-membrane, which is especially well marked in the serous membranes of man. The rest of the thickness of the membrane is composed of connective tissue, with a network of fine elastic fibres near the inner surface.

The cavities of the serous membranes are originally formed in the embryo as a cleft in the mesoblast (pleuro-peritoneal split) which becomes lined with epithelium, and its wall eventually becomes differentiated into the serous membrane.

The synovial membranes, which are often compared with the serous membranes, and are indeed, like the latter, connective-tissue membranes which bound closed cavities moistened with fluid, are not so intimately connected with the lymphatic system, nor is the fluid (synovia) which moistens them of the nature of lymph. Moreover, it is only here and there that there is a lining of epithelium-like cells, in place of the continuous lining of epithelium which we find in the serous membranes. Curious villus-like projections occur in many parts; they are covered by small rounded cells, and probably serve to extend the surface for the secretion of synovia. The blood-vessels of synovial membranes are numerous, and approach close to the inner surface of the membrane.