widely distributed throughout the body than the foregoing kind, is composed, like it, of cells and a matrix; the latter, however, being made up almost entirely of fibres closely resembling those of white fibrous tissue.

In this kind of fibro-cartilage

it is not unusual to find a great part of its mass composed almost exclusively of fibres, and deriving the name of cartilage only from the fact that in another portion, continuous with it, cartilage cells may be pretty freely distributed.

By prolonged boiling, cartilage yields a gelatinous substance called chondrin- white fibro-cartilage yields gelatin as well.

Functions of Cartilage.-Cartilage not only represents in the

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Fig. 50. White fibro-cartilage from an inter-vertebral ligament. (Klein and Noble Smith.)

foetus the bones which are to be formed (temporary cartilage), but also offers a firm, but more or less yielding, framework for certain parts in the developed body, possessing at the same time strength and elasticity. It maintains the shape of tubes as in the larynx and trachea. It affords attachment to muscles and ligaments; it binds bones together, yet allows a certain degree of movement, as between the vertebræ; it forms a firm framework and protection, yet without undue stiffness or weight, as in the pinna, larynx, and chest walls; it deepens joint cavities, as in the acetabulum, without unduly restricting the movements of the bones.

Development of Cartilage.-Cartilage is developed out of an embryonal tissue, consisting of cells with a very small quantity of intercellular substance: the cells multiply by fission within the cell-capsules (fig. 6); while the capsule of the parent cell becomes gradually fused with the surrounding intercellular substance. A repetition of this process in the young cells causes a rapid growth of the cartilage by the multiplication of its cellular elements and corresponding increase in its matrix. Thus we see that the matrix of cartilage is chiefly derived from the cartilage cells.

III. BONE.

Chemical Composition.-Bone is composed of earthy and animal matter in the proportion of about 67 per cent. of the former to 33 per cent. of the latter. The earthy matter is composed chiefly of calcium phosphate, but besides there is a small quantity (about

11 of the 67 per cent.) of calcium carbonate and fluoride, and magnesium phosphate.

The animal matter is resolved into gelatin by boiling.

The earthy and animal constituents of bone are so intimately blended and incorporated the one with the other, that it is only by chemical action, as, for instance, by heat in one case and by the action of acids in another, that they can be separated. Their close union, too, is further shown by the fact that when by acids the earthy matter is dissolved out, or, on the other hand, when the animal part is burnt out, the shape of the bone is alike preserved.

The proportion between these two constituents of bone varies in different bones in the same individual, and in the same bone at

different ages.

Structure. To the naked eye there appear two kinds of structure in different bones, and in different parts of the same bone, namely, the dense or compact, and the spongy or cancellous tissue.

Thus, in making a longitudinal section of a long bone, as the humerus or femur, the articular extremities are found capped on their surface by a thin shell of compact bone, while their interior is made up of the spongy or cancellous tissue. The shaft, on the other hand, is formed almost entirely of a thick layer of the compact bone, and this surrounds a central canal, the medullary cavity—so called from its containing the medulla or marrow.

In the flat bones, as the parietal bone or the scapula, one layer of the cancellous structure lies between two layers of the compact tissue, and in the short and irregular bones, as those of the carpus and tarsus, the cancellous tissue alone fills the interior, while a thin shell of compact bone forms the outside.

Marrow. There are two distinct varieties of marrow-the red and yellow.

Red marrow is that variety which occupies the spaces in the cancellous tissue; it is highly vascular, and thus maintains the nutrition of the spongy bone, the interstices of which it fills. It contains a few fat-cells and a large number of marrow-cells, many of which are undistinguishable from lymphoid corpuscles, and has for a basis a small amount of fibrous tissue. Among the cells are some nucleated cells of very much the same tint as coloured blood-corpuscles. There are also a few large cells with many nuclei, termed "giant-cells" (myeloplaxes), which are derived from over-growth of the ordinary marrow-cells (fig. 51).

Yellow marrow fills the medullary cavity of long bones, and consists chiefly of fat-cells with numerous blood-vessels; many of its cells also are in every respect similar to lymphoid corpuscles.

Fig. 51.-Cells of the red marrow of the guinea pig, highly magnified. a, a large cell, the nucleus of which appears to be partly divided into three by constrictions; b, a cell, the nucleus of which shows an appearance of being constricted into a number of smaller nuclei; c, a so-called giant cell, or myeloplaxe, with many nuclei; d, a smaller myeloplaxe, with three nuclei; e-i, proper cells of the marrow. (E. A. Schäfer.)

From these marrow-cells, especially those of the red marrow, are derived, as we shall presently show, large quantities of red bloodcorpuscles.

Periosteum and Nutrient Blood-vessels.-The surfaces of bones, except the part covered with articular cartilage, are clothed by a tough, fibrous membrane, the periosteum; and it is from the blood-vessels which are distributed in this membrane, that the bones, especially their more compact tissue, are in great part supplied with nourishment,-minute branches from the periosteal vessels entering the little foramina on the surface of the bone, and finding their way to the Haversian canals, to be immediately described. The long bones are supplied also by a proper nutrient artery which, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are distributed to the interior of the bone. Other small blood-vessels pierce the articular extremities for the supply of the cancellous tissue.

Microscopic Structure of Bone.-Notwithstanding the differences of arrangement just mentioned, the structure of all bone is found under the microscope to be essentially the same.

Examined with a rather high power its substance is found to

contain a multitude of small irregular spaces, approximately fusiform in shape, called lacune, with very minute canals or canaliculi, as they are termed, leading from them, and anastomosing with similar little prolongations from other lacunæ (fig. 52). In very thin layers of bone, no other canals than these may be

Fig. 52.-Transverse section of compact bony tissue (of humerus). Three of the Haversian canals are seen, with their concentric rings; also the corpuscles or lacunæ, with the canaliculi extending from them across the direction of the lamella. The Haversian apertures had got filled with débris in grinding down the section, and therefore appear black in the figure, which represents the object as viewed with transmitted light. The Haversian systems are so closely packed in this section, that scarcely any interstitial lamellæ are visible. X 150. (Sharpey.)

visible; but on making a transverse section of the compact tissue as of a long bone, e.g., the humerus or ulna, the arrangement shown in fig. 52, can be seen.

The bone seems mapped out into small circular districts, at or about the centre of each of which is a hole, and around this an appearance as of concentric layers-the lacunae and canaliculi following the same concentric plan of distribution around the small hole in the centre, with which, indeed, they communicate.

On making a longitudinal section, the central holes are found to be simply the cut extremities of small canals which run lengthwise through the bone, anastomosing with each other by lateral branches (fig. 53), and are called Haversian canals, after the name of the physician, Clopton Havers, who first accurately

described them.

53

The Haversian canals, the average diameter of

which is of an inch, contain blood-vessels, and by means of them blood is conveyed to all, even the densest parts of the bone; the minute canaliculi and lacunæ absorbing nutrient matter from the Haversian blood-vessels, and conveying it still more intimately to the very substance of the bone which they traverse.

The blood-vessels enter the Haversian canals both from without, by travers

ing the small holes which exist on the surface of all

bones beneath the perios- Fig. 53.-Longitudinal section of human ulna, show

teum, and from within by means of small channels which extend from the medullary cavity, or from the cancellous tissue. The arteries and veins usually occupy separate canals, and the veins, which are the larger, often present, at irregular intervals, small pouch-like dilatations.

The lacunae are occupied by branched cells (bonecells, or bone-corpuscles) (fig. 54), which very closely resemble the or dinary branched connec

ing Haversian canal, lacunse, and canaliculi.

(Rollett.)

tive-tissue corpuscles; each Fig. 54.-Bone corpuscles with their processes as seen in a thin section of human bone. (Rollett.) of these little masses of

protoplasm ministering to the nutrition of the bone immediately