244 HYALINE CARTILAGE. 



CARTILAGE. 



This is the well-known substance commonly called "gristle." The following are 

 its more obvious characters. When in mass, it is opaque and of a pearly or bluish 

 white colour, in some varieties yellow ; but in thin slices it is translucent. Although 

 it can be easily cut with a sharp knife, it is nevertheless of very firm consistence, but 

 at the same time highly elastic, so that it readily yields to pressure or torsion, and 

 immediately recovers its original shape when the constraining force is withdrawn. 

 By reason of these mechanical properties, it serves important purposes in the con- 

 struction of some parts of the body. 



In the early embryo the skeleton is, in great part, cartilaginous ; but the cartilage 

 forming its different pieces, which have the outward form of the future bones, in due 

 time undergoes ossification or gives place to bone, in the greater part of its extent at 

 least, and hence this variety of cartilage is named " temporary." 



Of the permanent cartilages a great many are in immediate connection with 

 bone, and may be still said to form part of the skeleton. The chief of these are the 

 articular and the costal cartilages ; the former cover the ends or surfaces of bones in 

 the joints, and afford these harder parts a thick springy coating, which breaks the 

 force of concussion and gives ease to their motions ; the costal or rib-cartilages form 

 a considerable part of the solid framework of the thorax, and impart elasticity to its 

 walls. Other permanent cartilages enter into the formation of the external ear, the 

 nose, the Eustachian tube, the larynx, and the windpipe. They strengthen the 

 substance of these parts without undue rigidity ; maintaining their shape, keeping 

 open the passages through them where such exist, and giving attachment to moving 

 muscles and connecting ligaments. 



Cartilages, except those of the joints, are covered externally with a moderately 

 vascular fibrous membrane named the perichondrium. 



When a very thin slice of cartilage is examined with the microscope, it is seen to 

 consist of nucleated cells, disseminated in a solid mass or matrix (fig. 285). The 

 matrix is sometimes transparent, and to all appearance homogeneous ; sometimes dim 

 and very faintly granular, like ground glass : both these conditions occur in hyaline 

 cartilage, which may be regarded as the most typical form of the tissue. Two 

 varieties exist in which the matrix is pervaded to a greater or less extent by fibres. 

 In the one, named elastic or yellow fibro-cartilage, the fibres are similar to those of 

 elastic tissue ; in the other, named white fibro-cartilage, they are of the white kind as 

 in ordinary ligament. 



HYALINE CAKTILAGE. 



Structure. In hyaline cartilage the matrix, as just stated, is uniform, and, 

 when examined fresh, usually appears free from fibres. Like the ground-substance 

 or matrix of connective tissue, it becomes stained brown by nitrate of silver and 

 subsequent exposure to the light. The cells consist of a rounded, oval, or bluntly 

 angular cell-body of translucent protoplasm, embedded in which are fine curvilinear 

 interlacing filaments and minute granules (fig. 284), with a round nucleus, which is 

 either clear with one or more nuclcoli, or, more commonly, is occupied by a network 

 of chromoplasm, which produces under a low power of the microscope a granular 

 effect. The cell-body lies in a cavity of the matrix, which, in its natural condition, 

 it entirely fills. This cavity is bounded and inclosed by a transparent capsule, which 

 is seldom obvious to the eye, for it coheres intimately with the surrounding matrix, 

 with which it agrees in nature, and cannot usually be distinguished without the aid 

 of re-agents. 



