CAETILAGE 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 articu- lar, are covered with a fibrous perichondrium, analogous to the periosteum of bones, which serve as support to the blood vessels. The sim- plest form of cartilage consists of nucleated cells, large, ovoid, more or less flattened by their mu- tual contact; the diminutive nucleus, attached to the cell wall, contains a minute nucleolus; these cells are scattered irregularly in an inter- cellular substance, or hyaline matrix, which contains numerous granules, many of which, ac- cording to Hassall, must be regarded as the cytoblasts from which new cells are developed ; the amount of this substance is greatest in the Homogeneous Substance and Cells of Cartilage. fully developed cartilage. In the condensed margin of true cartilage, the cells are com- pressed, with their long diameters parallel to the surface they cover; when ossification be- gins 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 hya- line matrix; they measure from j^t to ^5 of an inch; in their deep portions these car- tilages gradually blend with the bone, which dips unevenly into the substance of the car- tilage. In the cartilages of the ribs the cells are larger than in any other, being from -^J-y
- iiir f *** mcn m diameter; they often
have a linear arrangement, and are imbed- ded in a very abundant intercellular sub- stance, which sometimes presents a distinctly fibrous structure, though not resembling white fibrous tissues. In the membraniform carti- lages, the cells are very numerous in propor- tion 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 tu- mor attached to bone, containing cells like those of cartilage, and others of a peculiar form resembling the lacunae of bone. In the articulations, especially in the knee joint, loose rounded bodies are often found, of a cartila- ginous consistence, frequently as. large as the knee pan; these interfere with the motions of joints, and are sometimes removed by opera- tion. 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 impenetrated by a single vessel ; articular car- tilage is non-vascular, except in some diseased conditions when the presence of a few vessels seems to have been detected ; temporary car- tilage also, when in small mass, has no vessels, but when of considerable thickness the delicate extensions of the investing perichondrium pene- trate it in a tortuous manner ; the membrani- form resemble the temporary cartilages in re- spect to vascularity. The nutriment of articu- lar 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 de- velopment 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 develop- ment of secondary in parent cells, cartilages resemble the algce, 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 in- tercellular 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 ar- ranged in an intricate and interlaced manner, strongest in that direction in which the great- est toughness is required. To the strength of fibrous tissue is added the elasticity of carti- lage ; its vessels are few and derived from ad- jacent textures, and no nerves have been de- tected in it; its sensibility is low, and it has no vital contractility. The disks between the vertebrse are fibro-cartilage; their elasticity diminishes the shocks to which the spinal column is necessarily subjected ; in the whale
