By exposure to water and some other liquids, as well as to the action of electric shocks, the cell-body shrinks away from the inside of the capsule, and assumes a jagged or otherwise irregular figure, and then may hide the nucleus (fig. 287). It often contains larger or smaller fat-globules (fig. 285, g).

The cells of cartilage appear to contain glycogen, for they are coloured reddish brown by iodine (Neumann). They are rarely dispersed singly in the matrix; most commonly occurring in groups of two or more. When disposed in pairs (as at a, fig. 285) the cells are generally triangular or pyramidal in form with rounded angles,

IN THE LIVING STATE, FROM
THE SALAMANDER: HIGHLY

MAGNIFIED. (Flemming.)

and with their bases opposite one another; in the larger Fig. 284. A CARTILAGE CELL groups (b) the cells have a straight outline where they adjoin or approach one another, but at the circumference of the group their outline is rounded. Towards the surface of the cartilage the groups are generally flattened conformably with the surface, appearing narrow and almost linear when seen edgeways, as in a perpendicular section (fig. 286, a).

Various observers, and especially Tillmanns, have shown that the matrix of hyaline cartilage can be broken up after long maceration, and with the aid of

Fig. 285.-ARTICULAR CARTILLAGE FROM

HEAD OF METATARSAL BONE OF MAN

(OSMIC ACID PREPARATION). THE CELLBODIES ENTIRELY FILL THE SPACES IN THE MATRIX. 340 DIAMETERS.

(E. A. S.)

a, group of two cells; b, group of four cells; h, protoplasm of cell, with g, fatty granules; n, nucleus.

pressure, into fine fibrils. According to Cresswell Baber, these fibres are vertical to the surface in articular cartilage, and parallel with the long axis in rib cartilage. They are more easily seen in the cartilage of birds than of mammals. Their chemical nature is not very clear, nor is it certain how far the appearances correspond with any structure naturally present; but if, as Kühne and Merochowetz assert, gelatin and mucin can be obtained from the matrix of cartilage, the fibres in question may be chemically of the

same nature as the white fibres of connective tissue, the mucin belonging to the ground-substance in which they are embedded.

Other histologists have described a network of exceedingly fine ramified canals penetrating the cartilage-matrix, and effecting a communication between the cell-spaces. Up to the present time, however, the existence of such anastomosing channels has not been conclusively proved, although often assumed in order to explain the manner in which nutritive plasma penetrates the matrix of cartilage to reach the cells. Budge endeavoured to demonstrate the existence of canaliculi by forcing coloured injecting fluid into the substance of cartilage, but the result of the experiment was not conclusive. It has also been attempted to show them by the so-called natural method of injection, that is by allowing indigo-carmine (which has an

intensely blue colour) to mix with the circulating blood of animals, which after a time are killed and the cartilages examined. Proceeding in this way, Gerlach was unable to see any blue channels in the cartilage-matrix, while Arnold, on the other hand, obtained results from which he was led to infer the existence of minute cleft-like spaces throughout the matrix, connected by fine radiating canaliculi on the one hand with the lymphatics in the perichondrium, and on the other hand with the cell-spaces of the cartilage.

Such is the structure of hyaline cartilage in general, but it is more or less modified in different situations.

In articular cartilage, the matrix in a thin section appears dim, like ground glass, having sometimes an almost granular aspect. The cells and cell-groups are smaller and more dispersed, as a rule, than in rib-cartilage. As is the case also with the cartilage of the ribs, the groups are flattened at and near to the surface, and lie

Fig. 286.-VERTICAL SECTION OF ARTICULAR CARTILAGE COVERING THE LOWER END OF THE TIBIA

(HUMAN). MAGNIFIED ABOUT 30 DIAMETERS. (E. A. S.)

a, cells, and cell-groups flattened conformably with the surface; b, cell-groups irregularly arranged; c, cell-groups disposed perpendicularly to the surface; d, layer of calcified cartilage; e, bone.

parallel with it (fig. 286, a); deeper and nearer the bone, on the other hand, they are narrow and oblong, like short irregular strings of beads, and are mostly directed vertically (fig. 286, c). It is well known that articular cartilages readily break in a direction perpendicular to their surface, and the surface of the fracture appears to the naked eye to be striated in the same direction, as if they had a columnar structure; this may be due to the vertical arrangement of the rows of cells, or to the substance of the matrix being disposed in a fibrous or columnar manner (Leidy). it was formerly held that the free surface of articular cartilage is covered with epithelioid cells, but no such covering really exists. It is easy, no doubt, to peel off a thin film from the surface of the cartilage of the head of the humerus or femur; but this superficial layer is really part of the cartilage, and its broad patches of cells with the intermediate matrix are not to be mistaken (fig. 287).

Near the margin of the articular cartilages connective tissue is prolonged a certain way into them from the periosteum and synovial membrane, and the cartilagecells acquire processes and present transitions to the connective-tissue corpuscles of that membrane (fig. 288). There is no sharp demarcation between the two tissues, which here pass continuously into one another.

Except at this transitional zone the matrix of articular cartilage rarely

becomes converted into fibro-cartilage, nor is it prone to ossify like rib-cartilage. But a deposit of calcareous granules may occur in the deeper parts of the articular cartilage near the bone, the deposit first showing itself around the

Fig. 287.-A THIN LAYER PEELED OFF FROM THE SURFACE OF THE CARTILAGE OF THE HEAD OF THE HUMERUS, SHOWING FLATTENED GROUPS OF CELLS (Sharpey).

The shrunken cell-bodies are distinctly seen, but the limits of the capsular cavities where they adjoin one another are but faintly indicated. Magnified 400 diameters.

Fig. 288.-BORDER OF ARTICULAR CARTILAGE SHOWING TRANSITION OF CARTILAGE CELLS INTO CONNECTIVETISSUE CORPUSCLES. FROM HEAD OF METATARSAL BONE (HUMAN). ABOUT 340 DIAMETERS. (E. A. S.) a, ordinary cartilage cells; b,b, with branching processes.

groups of cartilage cells (fig. 286, d). This change may also happen at the symphyses. When the earthy matter is extracted by means of an acid, the tissue which remains has all the characters of cartilage.

In the costal cartilages, the cells, which are of considerable size, are also collected in groups, larger for the most part than those found in articular cartilage (fig. 289). Near the exterior of the cartilage they are flattened, and lie parallel with

Fig. 289.-FROM A SECTION OF COSTAL CARTILAGE FROM THE CALF. CHROMIC ACID PREPARATION (E. A. S.). The matrix is indistinctly fibrous. Two or three empty cell-spaces are seen in the section, the cells having dropped out. The cell-protoplasm is reticular.

the surface. As to those situated more inwardly, we can sometimes observe, in a transverse slice, that they form oblong groups disposed in lines radiating to the

circumference; but this arrangement is not constant, and they often appear quite irregular. The cells, with the exception of those lying upon the surface, frequently contain drops of oil, the nucleus being often altogether concealed by the fat. The matrix is clear, except where fibres have been developed in it, in which parts it is opaque and yellowish. Such fibrous patches are very frequent; the fibres are fine, straight, and parallel, appearing transparent when few together. Besides these fibrous patches in the interior of the rib-cartilages, the subperichondral layer is also pervaded by bundles of fibres which are directly prolonged from the fibre-bundles of the perichondrium and gradually lose themselves in the cartilage-matrix. There is in fact no sharp line of demarcation between the perichondrium and the subjacent cartilage, the one tissue passing by imperceptible gradation into the other. There is indeed reason to believe that the superficial layers of the cartilage are formed by a transformation of the fibrous tissue of the perichondrium during the growth of cartilage. It is not uncommon to find the rib-cartilages extensively ossified.

The description given of the microscopic characters of the costal cartilages will apply with little variation to the ensiform cartilage of the sternum, to the cartilages of the larynx and windpipe, except the epiglottis and cornicula laryngis, and to the cartilages of the nose. With the exception of the last, these resemble the ribcartilages also in their tendency to ossify.

The characters of the temporary cartilages, which are hyaline, will be noticed in the account of the formation of bone.

ELASTIC OR YELLOW CARTILAGE.

The epiglottis and cornicula of the larynx, the cartilages of the ear and of the Eustachian tube, differ so much from the foregoing, both in intimate structure and outward characters, that they have been included in a class apart, under the name of the "elastic," "yellow," or "spongy" cartilages. These are opaque and somewhat

Fig. 291.-SECTION OF PART OF THE CARTILAGE OF THE EPIGLOTTIS (Ranvier).

a, cartilage cell in clear area; b, granular-looking matrix near the middle of the cartilage, the granular appearance being due partly to the fine reticulum of elastic fibres, partly to the presence of granules of elastic substance in the matrix; c, clearer matrix with longer fibres.

yellow, are more flexible and tough than the ordinary cartilages, and have little tendency to ossify. They are made up of cells and a matrix, but the latter is everywhere pervaded with fibres (fig. 290), except in a small area or narrow zone left round each of the cells. The fibres resist the action of acetic acid; they are in many parts short, fine, and confusedly intersecting each other in all directions, like the

filaments in a piece of felt; in such parts the matrix has a rough indistinctly granular look, but sometimes this appearance is due to the fact that the elastic fibres are incompletely developed, the granules which are to form them having not yet run together into fibres. Sometimes the fibres are longer (fig. 291, c) but they still intercommunicate at short distances.

In large animals such as the ox, where the fibres of ordinary elastic tissue attain a considerable size, those of elastic cartilage are also very large with comparatively wide meshes, occupied of course by the hyaline ground-substance.

WHITE FIBRO-CARTILAGE.

This is a substance consisting of a mixture of the fibrous and cartilaginous tissues, and so far partaking of the qualities of both. Like hyaline cartilage, it possesses firmness and elasticity, but these properties are united with a much greater degree of flexibility and toughness. It presents itself under various forms, which may be enumerated under the following heads :

1. Interarticular disks. These are interposed between the moving surfaces of bones, or rather of articular cartilages, in several of the joints. In the joint of the lower jaw and in that of the clavicle they have the form of round or oval plates, growing thinner towards the centre; in the knee-joint they are curved in form of a sickle, and thinned away towards their concave free edge. In all cases their surfaces are free; while they are fixed by synovial or fibrous membrane at their circumference or extremities. The synovial membrane of the joint is prolonged for a short distance upon these fibro-cartilages, from their attached margin.'

2. The articular cavities of bones are sometimes deepened and extended by means of a rim or border of fibro-cartilage. Good examples of these marginal fibro-cartilages are seen in the shoulder and hip-joints, attached round the lip of the articular sockets. In the joint of the lower jaw, the cartilage lining the glenoid cavity is also largely fibrous.

3. Connecting fibro-cartilages are such as pass between the adjacent surfaces of bones in joints which do not admit of gliding motion, as at the symphysis of the pubis and between the bodies of the vertebræ. They have the general form of disks, and between the vertebræ are composed of concentric rings of fibrous tissue with cartilage-cells and matrix interposed; the fibrous tissue predominating at the circumference, the cartilaginous tissue increasing towards the centre. The bony surfaces which they connect are usually encrusted with true cartilage.

4. The bony grooves in which tendons of muscles glide are lined with a thin layer of fibro-cartilage. Small nodules of this tissue (sesamoid fibro-cartilages) may also be developed in the substance of tendons, of which there is an example in the tendon of the peroneus longus, and also in that of the tibialis posticus, where it passes beneath the head of the astragalus.

Fibro-cartilage appears under the microscope to be made up of wavy fibres, like those of ordinary ligament, with cartilage-cells occupying the place, and often simulating the arrangement, of the tendon-cells. As in elastic fibro-cartilage, the cells are immediately surrounded by a part of the matrix which is free from fibres (fig. 292). As a general rule they resemble the cells of ordinary cartilage, having a rounded shape, although somewhat flattened where the bundles of fibres are closely packed.

1 It has been stated by several authors that the interarticular disks are formed of fibrous tissue only, without any intermixture with cartilage. This statement is, however, incorrect. In all cases (jaw, clavicle and knee), there are unmistakeable rows and groups of cartilage-cells enclosed in capsules between the bundles of white fibres.