THE TISSUES 



33 



The cells containing the pigment are branched, and in 

 many cases they possess the power of movement. This is 

 specially well seen in such cells in the skin of the frog, where 

 contraction and expansion may be easily studied under the 

 microscope. By these movements the skin, as a whole, is 

 made lighter or darker in colour. The movements of these 

 cells are under the control of the central nervous system. 



3. Cartilage. While fibrous tissue is the great binding 

 medium of the body, support is afforded in foetal life and in 

 certain situations in adult life by cartilage. 



When cartilage is to be formed, the embryonic cells be- 

 come more or less oval, and secrete around them a clear 

 pellucid capsule. This may become hard, and persist 

 through life as in the so-called parenchymatous cartilage 

 of the mouse's ear. 



(1) Hyaline Cartilage. Development, however, usually 

 goes further, and before the capsule has hardened, the carti- 

 lage cells again divide, and each 

 half forms a new capsule which 

 expands the original capsule of 

 the mother cell, and thus in- 

 creases the amount of the 

 formed material. This formed 

 material has a homogeneous, 

 translucent appearance, and a 

 tough and elastic consistence, 

 and cuts like cheese with the 

 knife (Fig. 11). 



The formed material of car- 

 tilage is not a special substance, 

 but a mixture of chondroitin- 

 sulphuric acid with collagen in 

 combination with proteids. Chondroitin when decomposed 

 yields glucosamine, a sugar-like substance containing nitrogen, 

 and glycuronic acid, another substance closely related to the 

 sugars. 



Cartilage is surrounded by a fibrous membrane, the peri- 

 chondrium, and no hard and fast line of demarcation can 

 be made out between them. The fibrous tissue gradually 

 becomes less fibrillated the cells become less elongated and 



3 



FIG. 11. Hyaline Cartilage covered 

 by perichondrium. 



