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permit the passage of nutrient fluid from the bone to the deep surface of the cartilage. This is the one important point-it is immaterial what the channels, through which that fluid passes, be called, whether tubes, osseous intervals, or any other name, as long as it be fully understood, that there is a mechanism permitting fluid to permeate the structure and to nourish the cartilage from its deep surface.

That part of the epiphysis, which remains unossified towards the joint cavity is thus separated from the ordinary bone tissue, and now becomes articular cartilage. It is not, as it has often been said to be, attached to the bone, but it is continuous with it: it is part of the same thing, one portion having received a deposit of lime, the other portion not having done so. It varies in thickness according to the shape of the surface, and always so that its form is an exaggeration of that of the bone: thus, if the osseous surface be concave, the cartilage is thickest at its edges, so that it is more concave than the bone itself. If, on the contrary, we take a convex example, the cartilage will be found thinnest at the edges, so that the whole shape is more convex than the osseous surface.

Cartilage is a tough elastic material, of a semi-transparent bluish appearance, and easily cut with a knife. Treated with strong acetic acid it is dissolved into a jelly, which Müller called chondrin, and which differs very little from gelatine. If thin sections be made through the substance of the cartilage and examined by a quarter-inch power, the whole substance will be found to be very translucent, and to have a finely mottled or granular aspect; and there will be seen in it a great many bodies of an oval form. Some confusion has arisen from these having been named cartilage cells. The truth is, that they are cavities in the hyaline substance, having no lining membrane, which contain from two to six nucleated cells. I propose to call throughout this treatise each body, i.e., cells and hollow, the cartilage-corpuscle, reserving the word cartilage-cell for each one of the bodies contained in the cavity. The cells in the corpuscle not unfrequently, while near the attached surface, separate and cause a division by fissure of the hollow into new corpuscles, and the observer will find the arrangement of these bodies somewhat peculiar, for if he examine a fine vertical section of any articular cartilage in any animal, he will see those cartilage

corpuscles, which lie near the attached surface well developed, and containing each from two to six nucleated cells, and near this surface not only do the cells in each corpuscle, but also these latter bodies, tend to arrange themselves perpendicularly to the surface; and when a corpuscle divides it does so in the same direction. As the object is passed under the glass towards the free surface, he will be struck by a change in this respect; the cells no longer remain in the hollow so constantly perpendicular to one another, and as they divide, they do so as frequently horizontally as in any other direction; at last the divisions and the groupings all tend strongly to the horizontal; the cells themselves become separate, and are flattened in the same direction, till at last they become mere scales, three or four layers of which (fig. 5), lying close together, form the extreme free edge

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Fig. 5. Cartilage from human astragalus, magnified about 700 diameters, showing the perpendicular arrangement of corpuscles at the lower part, gradually curving

and drying into scales.

of the section; that is, the unattached surface of the articular cartilage consists of three or four layers of flattened cells lying quite close together and overlapping each other's edges. This ar rangement has caused many observers to believe in the existence of an epithelium. If the superficies of fresh cartilage be shaved off thin with a very sharp knife, the section will indeed have the appearance of a layer of epithelial cells; but if a thin slice through

into an oblique, subsequently into a horizontal, position, its substance be examined, the gradual horizontal arrangement and flattening of the cells will leave no doubt as to the true structure of its superficies.* (Fig. 5.)

* This arrangement has, however, not it is so difficult to procure sections sufbeen described as is here done, because | ficiently thin, that go all through the

Some observers, Mr. Toynbee among them, found, that in the foetus vessels run across the cartilage, even into the middle of joints. In neither a foetal hare or calf, that I had the opportunity of examining, could I discover any such arrangement, nor any trace of it in a stillborn child.* Nor have I been able to discover epithelium overlying the cartilaginous surface: what Mr. Bowman took for that structure was, I believe, the superficial layer of cells as above described, which, in the yet unused joint, is finer than when it has been subject to wear and tear.

The position of these cells and their gradual separation from each other, and diminution in size, prove that they derive their nutriment from the attached surface, and this is in consonance with the arrangement in other structures which line cavities, for they are all nourished by the deep surface. Thus the absence or presence of vessels upon the foetal cartilage is of importance, because if present it would establish the fact of a structure lining a cavity being nourished by its free surface. Besides, as it is clear that, at least during intra-uterine life, there is large provision for nutrition of the cartilage from the deep surface, the presence of such vessels would show that a structure having one free, one attached surface might be nourished from both. Moreover, it would prove that a cellular structure might be nourished from the surface towards which the cells grow. But the two first facts would be isolated, and the last is hardly conceivable; and as others besides myself have failed to discover this arrangement, there must have been in the observation of such vessels some occult source of error.

Moreover, the presence of a plexus beneath the articular lamella, and the structure of that bony plate, distinctly point out the attached surface as the source whence the whole is nourished. Thus a continual growth takes place from the attached towards the free surface, replacing the effects of wear and pressure, which are proved to be considerable, not only by unassisted reason, but also by our present knowledge of the flattening of cells on that surface.

attached surface, cutting slices with a well-made and sharp Valentin's knife.

cartilage, the outer layers of cells break- | cork, and beginning from the formerly ing easily from a very thin slice. It may best be managed by cutting away carti lage and articular lamella from the cancelli, scraping the osseous matter away, then laying the cartilage on a piece of

*Kölliker could not make out any vessels on the surface of foetal cartilages.

The synovial membrane, wrapping round each bone of the joint and extending from one to the other, shuts them into a cavity. The membrane, by and for itself, is not a closed sac, as has usually been described; but it is a tube, into the ends of which the bony and cartilaginous apparatus of the joint is inserted. It consists of a basement membrane, inside which is a layer of pavement epethelium cells, and on the outside a fine areolar tissue, known by the name of subsynovial, and which carries the vessels furnishing pabulum for nutrition and secretion to the part.

The mode in which the synovial membrane is continued over, or terminates upon, the cartilage, has been a matter of considerable controversy. Some persons have thought that it was prolonged over the face of the cartilage; others, that it went between that structure and the bone; and some even have supposed that it was split, and that a lamina was placed in either situation. Some believe that the membrane is prolonged over the cartilaginous surface until the movements of the joint wear it away. My own conviction is, that at no period is the cartilage covered by synovial tissue, but that the membrane becomes lost very near the edge of the cartilage. In no young animal will any structure be found like a fibrous, or even a homogeneous membrane on the cartilaginous face, and the last layer of cells cannot be regarded as appertaining to a prolonged synovial membrane, since it is very evidently the result of a gradual process of change, and is the arrangement, towards which all the corpuscles and cells tend even from their very deepest layers. Histologically the synovial membrane and the apparatus of bone and cartilage previously described are exact parallels; the former consists, as just related, internally of cells, then of a fine membrane permitting fluids to pass through from a vesselbearing subserous tissue. The whole cartilaginous apparatus consists of cells (with an intercellular substance), then of a fine lamella, permitting fluids to pass from a vessel-bearing tissue. These structures, or rather set of structures, being analogous, it is hardly conceivable that nature should permit such a tautology as to prolong the one over the other.

The membrane is then a tube, and in a simple joint, as the shoulder, has scarcely any arrangements worthy of being named

reflexions; from the edge of the cartilage on the humerus it is prolonged a little downwards on the periosteum, and is then doubled over, courses along, and lines the capsular ligament, till it reaches the other bone of the joint, where, turning again, it passes to a little beyond the cartilaginous end. In a joint, such as the knee, which possesses inter-articular cartilages and internal ligaments, the arrangement is a little more complicated, since the membrane, as it passes from femur to tibia, is interrupted in its course by the menisci, lines their upper then their under surfaces, being folded over the internal edge, and from its outer and lower corner is directed on to the other bone of the joint, the tibia; thus the only part of the meniscus not covered by the basement membrane and epethelium is its outer thick edge; and this is attached by continuity of fibre to the subserous tissue. The crucial ligaments are covered in the same way by folds from the back and front of the joint; and there exists, moreover, a fold that runs from the space between the condyles to the tissue below the patella; mucous ligament. It must be observed that the membrane in its course from bone to bone is not stretched tight, or bound close to the fibrous capsule; on the contrary, it hangs inwards in loose folds, which are marked in all joints, particularly in the knee, where some of them at the side of the ligamentum patella have been called alar ligaments. Such a fold fills in the sort of angular interval, which in most joints is left between the cartilages before they come in actual contact, and permits a plentiful development of loose subsynovial areolar tissue containing pellets of soft fat, and supporting a rich plexus of vessels, which, lying in this fold that encircles the joint, must run round with it, and was thus termed circulus vasculosus.

The inner surface is not merely smooth like that of an ordinary serous membrane: on the contrary, it is studded with a number of villous-like processes, synovial fringes. These are more closely placed and better developed on the loose folds than elsewhere; they are particularly well-marked in the knee over the lateral folds, alar and mucous ligaments, &c. These processes, smaller than intestinal villi, are like them, in as much as they are composed of vascular ramifications, covered by basement

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