The matrix immediately around the cartilage-cells is often marked. off from the rest by a concentric line or lines, this part of the matrix,

which is the latest formed, being known as the capsule of the cell. The cells are bluntly angular in form, the sides opposite to one another in

FIG. 60.-VERTICAL SECTION OF ARTICULAR CARTILAGE COVERING THE LOWER END

OF THE TIBIA, HUMAN. (Magnified about 30 diameters.)

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.

the groups being generally flattened. The protoplasm is very clear, but it may contain droplets of fat; and with a high power fine inter

lacing filaments and granules have been observed in it (fig. 59). During life the protoplasm entirely fills the cavity or cell-space which it occupies in the matrix; but after death, and in consequence of the action of water and other agents, it tends to shrink away from the capsule. The nucleus is round, and shows the usual intranuclear network.

In vertical section (fig. 60) the deeper cell-groups (c) are seen to be arranged vertically to the surface, the more superficial ones (a) parallel to the surface; whilst in an intermediate zone the groups are irregularly disposed (b). In the deepest part of the cartilage, next the bone, there is often a deposition of calcareous salts in the matrix (calcified cartilage, d).

The disposition of the cells of cartilage in groups of two, four, and so on, is apparently due to the fact that these groups have originated from the division of a single cell first into two, and these again into two, and so on (fig. 61). It would seem that the matrix is formed of

(Sharpey.)

A, cell in its capsule; B, divided into two, each with a capsule; c, primary capsule disappeared, secondary capsules coherent with matrix; D, tertiary division; E, secondary capsules disappeared, tertiary coherent with matrix.

successive portions, which are deposited around each cartilage-cell as the so-called 'capsules,' each newly formed portion soon blending in its turn with the previously formed matrix, whilst a new capsule is formed within it. The division of the cartilage-cell, like that of other cells, is accompanied by a process of karyokinesis.

Embryonic cartilage is characterised by the cells being usually more sharply angular and irregular, being even in some cases, markedly branched, like those which occur at the junction of cartilage and synovial membrane in the adult. The cells are also more closely packed, the matrix being in relatively less amount.

1. MAKE transverse and tangential sections of a rib-cartilage, which may either be fresh, or may have been preserved in spirit. Stain them with hæmatoxylin (if fresh, after treatment with acetic acid as in Lesson XI., §2), and mount in glycerine. Sketch a part of a transverse section under a low power and a cell-group from one of the tangential sections under a high power. Notice especially the arrangement of the cells, somewhat concentric near the surface but radial near the centre. The costal cartilages are often ossified near the middle in animals, but in man when ossification occurs it is the superficial layer which is invaded.

2. Make sections of the cartilage of the external ear, either fresh or after hardening in alcohol. Mount in dilute glycerine faintly coloured with magenta. If from the ox, notice the very large reticulating elastic fibres in the matrix. Notice also the isolated granules of elastin, and around the cartilage-cells the area of clear ground-substance. Draw a small portion of the section.

3. Mount a section of the epiglottis in the same way. Notice the closer network of much finer fibres in its cartilage.

4. Cut sections of white fibro-cartilage (intervertebral disk), which has been hardened in saturated solution of picric acid, followed by spirit, or in spirit only. Stain the sections with dilute hæmatoxylin. Mount in dilute glycerine. Observe the wavy fibres in the matrix and the cartilage-cells lying in clear areas often concentrically striated. Look for branched cartilage-cells. Sketch three or four cells and the adjoining fibrous matrix.

Costal cartilage. In the costal cartilages the matrix is not always so clear as in the cartilage of the joints, for it often happens that fibres become developed in it. The cells are generally larger and more angular than those of articular cartilage, and collected into larger groups (fig. 62). Near the circumference, and under the perichondrium or fibrous covering of the cartilage, they are flattened and parallel to the surface, but in the deeper parts they have a more irregular or a radiated arrangement. They frequently contain fat. The cartilages of the larynx and windpipe and of the nose resemble on the whole the costal cartilages, but the study of them may be deferred until the organs where they occur are dealt with.

Elastic or yellow fibro-cartilage occurs in only a few situations. These are, the cartilage of the external ear and that of the Eustachian tube, and the epiglottis and cartilages of Santorini of the larynx. The

FIG. 62.-SECTION OF RIB-CARTILAGE, SHOWING CELLS AND CELL-GROUPS IN A

Two or three empty cell-spaces are seen from which the cells have dropped out in preparing the section.

matrix is everywhere pervaded with well-defined branching fibres, which unite with one another to form a close network (fig. 63). These fibres resist the action of acetic acid, and are stained deeply by magenta; they are evidently elastic fibres. In the ox they are very large, but smaller in man, especially in the cartilage of the epiglottis (fig. 64). They appear to be developed, as with elastic tissue elsewhere (see p. 49), by the deposition of granules of elastin in the matrix, which at first lie singly, but afterwards become joined to form the fibres.

White fibro-cartilage is found wherever great strength combined with a certain amount of rigidity is required: thus we frequently find fibro-cartilage joining bones together, as in the case of the intervertebral disks and other symphyses. Fibro-cartilage is frequently employed to line grooves in which tendons run, and it may also be found in the tendons themselves. It is also employed to deepen cup-shaped articular surfaces; and in the case of the interarticular cartilages, such as those of the knee and lower jaw, to allow greater freedom of movement whilst diminishing the liability to dislocation. Under the microscope white fibro-cartilage looks very like fibrous tissue, but its cells are

cartilage-, not tendon-, cells (fig. 65). They are rounded or bluntly angular and surrounded by a concentrically striated area of clear

a

FIG. 63.-SECTION OF THE ELASTIC CARTILAGE OF THE EAR. (Hertwig.) (Highly magnified.)

FIG. 64.-SECTION OF PART OF THE CARTI-
LAGE 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.

cartilage-matrix. In some parts of the intervertebral disk many of the

FIG. 65.-WHITE FIBRO-CARTILAGE FROM AN INTERVERTEBRAL DISK, HUMAN. (Highly magnified.)

The concentric lines around the cells indicate the limits of deposit of successive capsules. One of the cells has a forked process which extends beyond the hyaline area surrounding the cell, amongst the fibres of the general matrix.

cells are branched, and may be looked upon as transitional forms to connective-tissue corpuscles.