FIBROUS TISSUE 



517 



Where a connective tissue comes to a surface, as where it comes 

 in contact with an epithehum, its constituents are usually con- 

 densed to form a sheet of material called a basement membrane. 



CARTILAGE 



More specialised mechanical tissues are the skeletal tissues, 

 and of these there are two : cartilage and bone. Both are charac- 

 terised by a large amount of tough ground substance, but this, 

 as well as the type of cell, is different in the two tissues. The 

 simplest type of cartilage, called hyaline on account of its clear 

 or glassy appearance, is one of the 

 easiest of all tissues to recognise. 

 There are characteristic oval cells 

 (Fig. 402), sometimes in groups of 

 two or four where they have 

 recently divided, and each cell or 

 group is surrounded by a newly 

 formed capsule of chondrin, which 

 merges into the older chondrin 

 farther away. Cartilage cells are 

 derived from chondroblasts, which 

 in their early stages are indis- 

 tinguishable from fibroblasts. 

 Some division of the cells goes on 

 inside the tissue, but most of its 

 growth takes place on the surface, 

 where chondroblasts form a layer, 

 the perichondrium. Hyaline car- 

 tilage, in spite of its appearance, does contain many collagenous 

 fibrils, which, however, are not easily made visible. In adult 

 mammals it is found only in a few places, such as the respiratory 

 tract, where it makes the rings of the tubes, the ends of the ribs, 

 and covering the articular surfaces of bones, but in the embryo, 

 as well as in the adult frog and dogfish, there is much more of it. 

 Fibrocartilage may be regarded either as cartilage in which 

 there are many and conspicuous white fibres, or as fibrous tissue 

 in which small patches of cartilage have developed. It is found 

 between bones, as in the intervertebral discs, and sometimes 

 in tendons. Elastic cartilage contains many yellow fibres ; it 

 is found in the epiglottis and Eustachian tubes, and especially 



Fig. 402. — Cartilage stained and 

 magnified, showing cells, some 

 of which are in pairs formed by 

 the division of a single cell, 

 matrix, and the newly secreted 

 part of the matrix, which forms 

 capsules around the cells. 



