Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/906

Rh 852 ANATOMY termed permanent. The following varieties of cartilage, based on modifications in structure and appearance, may be recognised Celluiar cartilag Cells with matrix substance. Matrix homogeneous. (Hyaline cartilage.) Matrix fibrous. (Fibro-cartilage. ) White nbro-cartikge. Yellow fibro-cartilage. The Cellular or Parenclnjmatous Cartilage does not exist in the adult human body. It occurs, however, in the human embryo, in the embryos of all the vertebrata, and in the larval stage of development of the tunicata, as the slender rod named chorda dorsalis or notochord. In all the higher vertebrata the chorda dorsalis disappears as development advances, but in the lower vertebrates it per sists throughout life as a more or less perfect structure. In the lamprey and myxine it forms a continuous rod in the vertebral region. In fish gene rally, but more especially in the cartilaginous group, it forms a jelly-like mass, occupying the concavities between the bodies of the vertebrae. The cells lie in contact with each other. They are comparatively large in size, are sometimes rounded, but more usually compressed late ** a s2Sfi55S to * rally. The nucleus is often very distinct, though at other times more difficult to detect, and the cell wall is well marked. Sometimes a little inter cellular substance is found. By some anatomists the chorda dorsalis is regarded as a variety of connective tissue, and not of cartilage The cartilaginous framework of the ear of some small mammals as the mouse, the bat, and the rat is formed of cellular cartilage, the cells of which are smaller in size than those of the chorda dorsalis, irregularly poly gonal, and closely packed together so as to form a solid tissue. The Hyaline Cartilage consists of cells imbedded in a pellucid or hyaline matrix, which, under some conditions, however, may assume a dimly granulated appearance. The xiphoid and costal cartilages, the encrusting cartilages at the articular ends of the bones, the cartilages of the nose, those of the windpipe, except the epiglottis and cornicula laryngis, belong to this var iety, as also the temporary car tilages. In hyaline cartilage the cells are ovoid or poly gonal, or even fusiform, and sometimes flattened, the flat tened form of cell being found next the surface of the cartilage. They lie singly, or in groups of two, or three, or four ; sometimes they are arranged in linear series, at other times they are irregularly grouped together. The cell contents are dimly granular, with a well-defined nucleus containing a nucleolus. Not unfrequently two or more nuclei are present in a cell ; and in old cartilage the con tents are often coarsely granular, or even infiltrated with drops of oil. Heidenhain has shown that powerful induc tion shocks cause contraction of the protoplasm of the cells towards the central nucleus. The cells lie in cavities in the matrix substance, and the part of the matrix which forma the immediate wall of the hollow is named the Fio. 46. Cells of the cartilagi nous framework of the ear of the mouse. FIG. 47. Hyaline costal cartilage. [ANATOMY OF Tvo or more cells may sometimes lie FJG. 48. Vertical section through an encrusting cartilage. B, the bone on which the cartilage rests. capsule of the cell. in the same hollow. The matrix of hyaline cartilage is usually homogeneous. In some animals the matrix appears to have a concentric arrangement around the cells; and Rollett has stated that by the use of dilute sulphuric acid or chromic acid the matrix may be made to split up into concentric layers. Sometimes the matrix appears granulated, a change which is very apt to occur in sections of cartilage which have been removed for some time from the body. In the costal cartilages of old persons the matrix becomes fibrous; and it is by no means uncommon to find in advanced age these bars of cartilage converted into bone. In the articular or encrusting cartilages the arrangement of the cells is quite distinctive. If a vertical section be made through a plate of this carti lage, the cells next the bone are seen to be arranged in parallel rows perpendicular to the surface of the bone on which the cartilage rests; the cells are smaller than those of the costal cartilage, oblong in form, and the adjacent rows are separated by intermediate hyaline matrix. Near the free surface of the cartilage the cells are flattened, placed parallel to the plane of the surface, and so closely packed together that the proportion of matrix is much re duced. In the intermediate parts of the cartilage the cells lie irregularly in the matrix, and are rounded in form. It was from the study of the changes which take place in articular cartilage in disease that Goodsir was enabled to establish the production of new cells by the multiplication of the normal pre-existing cells of the cartilage, an observation which formed the starting-point of the modern doctrine of cellular pathology. Fibro-cartilages are divided into white and yellow. White fibro-cartilage may form the connecting medium between the articular surfaces of an amphiarthrodial joint, as in the inter- vertebral discs; or it may form plates in the interior of joints, as in the semi- lunar cartilages of the knee and other menisci in diarthrodial joints; or it may extend around the margin of the socket of a joint, as in the cotyloid ligament of the hip; or it may invest the surfaces of bones over which tendons have to play, as where the tendons of the peronei muscles play in the groove on the back of the external malleolus. In the intervertebral disco, which give the best illustrations of the structure of white fibro-cartilage, the cells are ovoid in form and distinctly nucleated. Sometimes two or three are grouped together, but not unfrequently they occur singly. They are sepa rated from each other by short fibres. In these discs the fibrous matrix is always stronger and more distinct in the peripheral than in the central part. The other forms of white fibro-cartilage are transitional between the true cartilage and connective tissue, i.e., the cells possess the characters of cartilage cells, whilst the fibrous matrix approximates to the matrix of the connective tissue. The yellow elastic fibro-cartilages are the epiglottis, the cornicula laryngis, the cartilaginous framework of the auricle of the human ear, and the ears of mammalia generally, and the cartilaginous wall of the Eustachian tube. The cells are rounded or ovoid, distinctly nucleated, and usually arranged singly or in pairs. The matrix is distinctly fibrous; the fibres, which form a close intersecting net- F IO. 49. White fibro-cartilage of an intervertebraJ disc.