substance or by processes which are sent up to the surface of the membrane from more deeply lying cells (pseudostomata of Klein and Burdon-Sanderson): the larger ones, on the other hand, are true apertures (stomata), which are surrounded by a ring of small cubical cells (fig. 449, s, s'), and open into a subjacent lymphatic vessel, either directly or by the medium of a short canal lined with similar cells. The surface cells of the serous membrane are not everywhere uniform in size, but patches are here and there met with in which they are smaller and more granular in appearance and it is in these parts that the stomata and pseudo-stomata are more frequently seen (figs. 448, 449). The epithelium-cells of the membrane often

Fig. 448.-PORTION OF ENDOTHELIUM OF PERITONEUM FROM THE UNDER SURFACE OF THE RABBIT'S DIAPHRAGM. (Klein.)

a, larger cells; b, smaller ones, with here and there a pseudostoma between.

present a somewhat radiated aspect near the stomata, the silver lines converging towards the orifice. According to Klein, it is not unfrequent to find evidences of proliferation, especially in the neighbourhood of the stomata and pseudo-stomata, cells being met with containing two or even many nuclei, and others which are being budded off from the cells of the membrane. Since, however, these statements depend on observations which were made before the importance of the karyokinetic figures as a guide to cell-multiplication was recognised, it is important that they should be repeated.

The stomata were discovered in the peritoneal covering of the central tendon of the diaphragm by Recklinghausen, who found that milk-globules could be made to pass through them into the lymphatics. Similar apertures were found by Ludwig and Dybkowsky in the pleura of mammals, and by Schweigger-Seidel and Dogiel in the septum between the peritoneal cavity of the frog and the great lymph-sac (cisterna magna) behind it. They have since been discovered on the omentum by Klein, and have also been recognised in the pericardium.

The substance of the membrane underneath the endothelium is composed of a connective tissue ground-substance in which is a variable amount of fibres, both white and elastic; the latter in many serous membranes, as remarked by Henle, are principally collected into a reticular layer near the surface. The bundles of white fibres are also arranged in a reticular manner, frequently uniting with one another, and the meshes of the reticulation which they form are occupied by the groundsubstance of the membrane, and bridged over by the flattened cells of the general

surface. In some folds of the serous membranes and especially in the great omentum of many animals, including man, the meshes of the reticulation have become open in many parts owing to the absorption of the intervening groundsubstance and the perforation of the cells covering it, so as to allow of a free communication between the two sides of the fold of membrane. Where the membrane is thicker, the ground-substance contains blood-vessels and lymphatics, with the

7, lymph-channel below the surface, lying between tendon bundles, t, t, 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. lymphoid and adipose tissue which is so often found in the serous membranes and especially in their folds; as well as connective tissue corpuscles with their corresponding cell-spaces (figs. 438, 439), which in the serous membranes are very often collected into endothelium-like patches. In parts of the membrane in which the corpuscles are more

thinly scattered, they possess branching processes, some of which intercommunicate with those of neighbouring cells, others may pass up to the surface of the membrane as pseudostomata and others again become connected to the walls of the lymphatics and blood-vessels.

In the human subject, the serous membranes are bounded under the epithelium by a distinct basement membrane (Bizzozero).

The blood-vessels of the membrane end in a capillary network with comparatively wide meshes, which pervades the subserous tissue and the tissue of the serous membrane. The vessels are much more numerous in the nodules and tracts of lymphoid tissue (see below) as well as in the adipose tissue, which is found largely developed in the serous membranes of fat animals.

The lymphatics of the serous membranes are exceedingly abundant. Their relation both to the cell-spaces of the tissue and to the surface of the membrane, as well as their general arrangement, has been already noticed. They are sometimes met with ensheathing the blood-vessels.

Nodules of lymphoid tissue may occur, as before mentioned (p. 387), in the substance of the serous membranes. More generally the lymphoid tissue of the serous membranes takes the shape of elongated tracts which follow the course of the small arteries and veins, receiving from the latter branches which divide to form a capillary network. Lymphatic vessels run in these tracts alongside the bloodvessels, and often partially enclose them. These lymphoid nodules and tracts are more numerous in the young animal; in the adult they are frequently found transformed into lobules and tracts of adipose tissue.

The nerves of the serous membranes are destined chiefly for the blood-vessels, and for the most part accompany these in their course. A few pale fibres, however, are distributed to the substance of the membrane, in which they form a plexus with large meshes from the branches of this plexus, fibrils may be traced which unite into a somewhat finer plexus near the surface.

SYNOVIAL MEMBRANES.

These are connective tissue membranes which are found surrounding closed cavities in connection with moveable structures in certain parts, such as the joints, the elongated sheaths in which some tendons glide, and at various situations between the skin and bony prominences below it. Although they resemble serous membranes in some respects, the synovial membranes are distinguished by the nature of their secretion, which is a viscid glairy fluid resembling the white of an egg and named synovia. From its nature it is well adapted for diminishing friction, and thereby facilitating motion.

If a drop of synovial fluid is examined microscopically, it is found to contain (in addition to fat-molecules) a few amoeboid corpuscles, as well as cells similar to those which occur on the projections of the membrane.

The different synovial membranes of the body are referred to three classes, viz., articular, vaginal, and vesicular.

1. Articular synovial membranes, or Synovial capsules of joints.-These by their secretion lubricate the cavities of the diarthrodial articulations, that is, those articulations in which the opposed surfaces glide on each other. In these cases the membrane may be readily seen covering internally the surface of the capsular and other ligaments which bound the cavity of the joint, and affording also an investment to any tendons or ligaments which pass through the articular cavity, as in the instance of the long tendon of the biceps muscle in the shoulder-joint. On approaching the articular cartilages the membrane does not pass over them, but terminates after advancing but a little way on their surface, with which it is here firmly adherent. The synovial membranes, therefore, do not form closed bags lying between the articular cartilages as was supposed by the older anatomists, for the main part of the surfaces of the joints is not covered at all by the membrane, nor even by a layer of epithelium-cells, prolonged from the membrane, as some have described.

In several of the joints, folds of the synovial membrane pass across the cavity; these have been called synovial ligaments. Other processes of the membrane simply project into the cavity at various points. These are very generally cleft into fringes at their free border, upon which their blood-vessels, which are numerous, are densely distributed. The larger folds and processes often contain fat, and then are sufficiently obvious; but many cf the folds are small and inconspicuous.

The fringed vascular folds of the synovial membrane were described by Havers in 1691, under the name of the mucilaginous glands, and he regarded them as an apparatus for secreting synovia. Rainey found that these Haversian fringes, as they are sometimes called, may exist in all kinds of synovial membranes, and that from the primary vascular fringes other smaller secondary processes are sent off, into which no blood-vessels enter.

2. Vaginal synovial membranes, or Synovial sheaths.-These are intended to facilitate the motion of tendons as they glide in the fibrous sheaths which bind them down against the bones in various situations. The best-marked examples of such fibrous sheaths are to be seen in the hand and foot, and especially on the palmar aspect of the digital phalanges, where they confine the long tendons of the flexor muscles. In such instances one part of the synovial membrane forms a lining to the osseo-fibrous tube in which the tendon runs, and another part affords a close investment to the tendon. The space between these portions of the membrane is lubricated with synovia and crossed obliquely by one or more folds or duplications of the membrane named "fræna," in some parts inclosing a considerable amount of elastic tissue (J. Marshall).

3. Vesicular or Bursal synovial membranes, Synovial bursa, Bursa mucosa. In these the membrane has the form of a simple sac, interposed, so as to prevent friction, between two surfaces which move upon each other. The synovial sac in such cases is flattened and has its two opposite sides in apposition by their inner surface, which is free and lubricated with synovia, whilst the outer surface is attached by areolar tissue to the moving parts between which the sac is placed.

In regard to situation, the bursæ may be either deep-seated or subcutaneous. The former are for the most part placed between a muscle or its tendon and a bone or the exterior of a joint, less commonly between two muscles or tendons: certain of the bursæ situated in the neighbourhood of joints not unfrequently open into them. The subcutaneous bursæ lie immediately under the skin, and are found in various regions of the body interposed between the skin and some firm prominence

B. PORTION OF THE SURFACE OF A VAGINAL SYNOVIAL MEMBRANE, AFTER TREATMENT WITH
NITRATE OF SILVER. 250 DIAMETERS. (E. A. S.)

The cell-spaces of the tissue and the nuclei of the contained cells only are represented. e, epithelioid arrangement of cells; s, ramified cells.

beneath it. The large bursa situated over the patella is a well-known example of this class, but similar though smaller bursæ are found also over the olecranon, the malleoli, the knuckles, and various other prominent parts. It must, however, be observed that, among these subcutaneous bursæ, some are reckoned which do not always present the characters of true synovial sacs, but look more like mere recesses in the subcutaneous areolar tissue, larger and more defined than the neighbouring areolæ, but still not bounded by an evident synovial membrane. These may be looked on as examples of less developed structure, forming a transition between the areolar tissue spaces and perfect synovial cavities; indeed it may happen that what is a well developed synovial bursa in one subject is merely an enlarged areola in another. Many of the bursæ do not appear until after birth, and they are said to increase in number as age advances.

Structure of synovial membranes.-The synovial membranes are composed entirely of connective tissue with the usual cells and fibres of that tissue. It was formerly stated, and is still asserted by some authors, that they are lined with an

epithelial layer of flattened cells, similar to those lining the serous membranes, but, as was shown by Hüter, there exists on the synovial membranes no complete lining of the kind. Patches of cells may, it is true, here and there be met with which present an epithelioid appearance (fig. 450, B, e), as, indeed, we know to be the case in the connective tissue of other parts; but most of the surface-cells of the synovial membranes are of the irregularly-branched type (fig. 450, A), the surface of the membrane between the cells and sometimes also over them being formed by the ground-substance of the connective tissue, whilst here and there small blood-vessels come close to the surface from subjacent parts. The cells are in many places smaller than in connective tissue generally. They vary much in shape in different parts; sometimes forming a network in the tissue by the anastomoses of their ramifying processes, in other parts being rounded, and more closely arranged.

The cells of the vaginal synovial membrane are often slightly elongated in the direction of the axis of the tendon.

The articular synovial membranes pass, as before said, a certain distance over the cartilages of the joints. They do not, however, end abruptly, but shade off gradually

Fig. 451.-TRANSITION OF CARTILAGE-CELLS INTO CONNECTIVE-TISSUE CORPUSCLES OF SYNOVIAL MEMBRANE. ABOUT 340 DIAMETERS. FROM HEAD OF METATARSAL BONE, HUMAN. (E. A. S.)

a, ordinary cartilage cells; b, b, with branched processes.

into the margin of the cartilage, the fibrous tissue becoming fibro-cartilage and the cells gradually losing their processes and becoming transformed into cartilage-cells (fig. 451), so that it is difficult to say where the one structure begins and the other ends. This portion of the synovial membrane, or of the cartilage, is known as the "marginal zone;" it is best marked around the convex heads of the bones, and is especially well seen near the lower margin of the patella (Hüter).

The Haversian folds and fringes, at least the larger ones, agree in general structure with the rest of the tissue of the synovial membrane, except that, as before remarked, some of them contain fat; their surface layer contains for the most part irregularly stellate cells, except over the fat, where there is occasionally to be observed a true epithelioid covering like that of a serous membrane. The smaller non-vascular secondary fringes of Rainey (synovial villi) are minute fingershaped processes projecting from the margins of the larger ones, and consist for the most part of small rounded cells with granular protoplasm and but little intercellular substance, enclosing a central strand of connective-tissue fibrils; and in some cases a few cartilage-cells. Some of the synovial villi are entirely made up of fibro-cartilage, being altogether destitute of the covering of rounded cells (Tillmanns).

Vessels and lymphatics.-The blood-vessels in and immediately beneath the synovial membrane are numerous in most parts of the joints. They advance but a