Cartilage, a firm, elastic substance, of an apparently homogeneous structure, bearing some analogy to bone, and entering largely into the composition of the animal skeleton; in its intimate structure it approaches very closely the cellular tissues of vegetables. It constitutes the rudimentary skeleton of the higher mollusca, and of the selachian fishes, hence called cartilaginous fishes; in man and the higher animals it forms the internal skeleton at the early periods of life, and is in all employed as a nidus for the development of bone. The organic basis of cartilage is a variety of gelatine called chondrine; this, like gelatine, in a watery solution solidifies on cooling, and may be precipitated by alcohol, creosote, tannic acid, and corrosive sublimate, and is not precipitable with ferrocyanide of potassium; but, unlike gelatine, it is precipitable with acetic and the mineral and other acids, with alum, persulphate of iron, and acetate of lead. True cartilage is of a white or bluish white appearance; fibro-carti-lage is of a yellowish color, and exhibits a fibrous structure.
Temporary cartilages supply the place of bone in early life, and gradually become ossified; for a considerable time after birth the ends of the long bones are composed chiefly of cartilage, and the extremities are not united to the shafts by bone until about the 20th year. Permanent cartilages are divided into two kinds, the articular and the membraniform; the skeleton of the selachians is also permanent cartilage. Articular cartilages cover the ends of bones entering into the formation of joints, either a thin layer between almost immovable bones, as those of the cranium, ilium, and sa-crum, or incrusting the ends in the free-moving ball-and-socket and hinge joints. The membraniform cartilages have no relation to locomotion, but serve to keep open canals or passages by the mere force of their elasticity: such are the cartilages of the external ear, nose, edge of the eyelids, Eustachian tube, and the air passages. The distinguishing characters of cartilage are elasticity, flexibility, and cohesive power; it is not easily broken, and will speedily resume its proper shape when bent by accident or design. These varieties of cartilage, except the articular, are covered with a fibrous perichondrium, analogous to the periosteum of bones, which serve as support to the blood vessels.
The simplest form of cartilage consists of nucleated cells, large, ovoid, more or less flattened by their mutual contact; the diminutive nucleus, attached to the cell wall, contains a minute nucleolus; these cells are scattered irregularly in an intercellular substance, or hyaline matrix, which contains numerous granules, many of which, according to Hassall, must be regarded as the cytoblasts from which new cells are developed; the amount of this substance is greatest in the fully developed cartilage. In the condensed margin of true cartilage, the cells are compressed, with their long diameters parallel to the surface they cover; when ossification begins in temporary cartilage, the cells become disposed in rows, as described in the article Bone. In the articular cartilages the cells are arranged in small groups in an abundant hyaline matrix; they measure from 1/1300 to 1/900 of an inch; in their deep portions these cartilages gradually blend with the bone, which dips unevenly into the substance of the cartilage.
In the cartilages of the ribs the cells are larger than in any other, being from 1/650 to 1/430 of an inch in diameter; they often have a linear arrangement, and are imbedded in a very abundant intercellular substance, which sometimes presents a distinctly fibrous structure, though not resembling white fibrous tissues. In the membraniform cartilages, the cells are very numerous in proportion to the intercellular substance, which is so fibrous in its character in the external ear as to approach very near to fibro-cartilage; the ear of the mouse is a good specimen of this form, and presents in its central portion a series of six-sided cells arranged in layers one above the other, resembling, except in size, the transverse section of the pith of a plant. Cartilage is sometimes found as an accidental and diseased product. Enchondroma is a tumor attached to bone, containing cells like those of cartilage, and others of a peculiar form resembling the lacunar of bone. In the articulations, especially in the knee joint, loose rounded bodies are often found, of a cartilaginous consistence, frequently as large as the knee pan; these interfere with the motions of joints, and are sometimes removed by operation.
The cartilage cells of reptiles are larger than those of fishes, being largest in the siren; in birds cartilage is very early converted into bone, so that they have very little of it except in the joints; the largest cells in the mammals, according to Mr. Quekett, are found in the elephant. Cartilage belongs to non-vascular substances, as considerable masses are found unpenetrated by a single vessel; articular cartilage is non-vascular, except in some diseased conditions when the presence of a few vessels seems to have been detected; temporary cartilage also, when in small mass, has no vessels, but when of considerable thickness the delicate extensions of the investing perichondrium penetrate it in a tortuous manner; the membraniform resemble the temporary cartilages in respect to vascularity. The nutriment of articular cartilage is derived from the vessels of the joint, and from the synovial membrane, though none of these enter its substance, the nutrient material passing from cell to cell by imbibition; in cartilages of ossification vessels regularly appear, accompanying the process of bone formation.
According to Hassall, cartilage cells are multiplied in two ways: 1, by the division of a single cell into two or more parts, each becoming a distinct cell; 2, by the development of cytoblasts in the intercellular substance, or in the parent cells, constituting a true reproduction, constantly going on. In this multiplication by division, and by development of secondary in parent cells, cartilages resemble the algae, and herein they stand alone in the animal economy. Cartilage cannot be regenerated; fractured surfaces are united only by a condensed cellular tissue. - There is a form of tissue which may be described here, as it differs from cartilage chiefly in having its intercellular substance replaced by white fibrous tissue; it is therefore called fibro-cartilage. It occurs principally in the joints, where its strength and elasticity are most needed. Its color is white, slightly tinged with yellow, with the shining fibres of the white fibrous tissue quite conspicuous; its consistence varies from pulpy to very dense. The fibres are arranged in an intricate and interlaced manner, strongest in that direction in which the greatest toughness is required.
To the strength of fibrous tissue is added the elasticity of cartilage; its vessels are few and derived from adjacent textures, and no nerves have been detected in it; its sensibility is low, and it has no vital contractility. The disks between the vertebra) are fibro-cartilage; their elasticity diminishes the shocks to which the spinal column is necessarily subjected; in the whale these disks are very large, detached from the vertebral bodies, and more or less ossified. In the diarthrodial joints, as in the sterno-clavicu-lar, temporo-maxillary, and knee joints, there are fibrous laminae, free on both surfaces, called menisci; in these the circumference is fibro-cartilage, and the centre more cartilaginous. On the edges of the shoulder and hip joints is a rim of fibro-cartilage, giving depth to the articular cavities. In the grooves in bone for the lodgment of tendons we find another instance of the occurrence of fibro-cartilage. Fibro-cartilage is not so prone to ossification as the simple fibrous structures; it is repaired by a new substance of similar texture; in cases of falsi' joint from the non-union of fractured bone, the broken ends are sometimes connected by fibro-cartilage. The pubic bones at the symphysis are united by this tissue.
Fibro-cartilage is less soluble in boiling water than true cartilage, and yields therefore less chon-drine. - The uses of cartilage and fibro-cartilage are entirely of a mechanical nature; their structure is admirably adapted for the protection of organs by their solidity, flexibility, and elasticity.
Homogeneous Substance and Cells of Cartilage.