THE STRUCTURE OF THE ELEMENTARY TISSUES. 55 



of cartilage only from the fact that in another portion, continuous with 

 it, cartilage cells may be pretty freely distributed. 



By prolonged boiling, cartilage yields a substance called chondrin 

 which gelatinizes on cooling. The cells of white fibre-cartilage are as a 

 rule rounded or somewhat flattened but in some places are distinctly 

 branched. 



Functions of Cartilage. Cartilage not only represents in the 

 foetus the bones which are to be formed (temporary cartilage) but also 

 offers a firm, yet more or less yielding, framework for certain parts in 

 the developed body, possessing at the same time strength and elasticity. 

 It maintains the shape of tubes as in the larynx and trachea. It affords 

 attachment to muscles and ligaments; it binds bones together, yet allows 

 a certain degree of movement, as between the vertebrae; it forms a firm 

 framework and protection, yet without undue stiffness or weight, as in 

 the pinna, larynx, and chest walls; it deepens joint cavities, as in the 

 acetabulum, without unduly restricting the movements of the bones. 



Development of Cartilage. Cartilage is developed out of an em- 

 bryonal tissue, consisting of cells with a very small quantity of intercel- 

 lular substance: the cells multiply by fission within the cell- capsules, 

 while the capsule of the parent cell becomes gradually fused with the 

 surrounding intercellular substance. A repetition of this process in the 

 young cells causes a rapid growth of the cartilage by the multiplication 

 of its cellular elements and corresponding increase in its matrix. Thus 

 we see that the matrix of cartilage is chiefly derived from the cartilage 

 cells. 



III. BONE. 



Chemical Composition. Bone is composed of earthy and animal mat- 

 ter in the proportion of about 67 .per cent of the former to 33 per cent 

 of the latter. The earthy matter is composed chiefly of calcium phos- 

 phate, but besides there is a small quantity (about 11 of the 67 per cent) 

 of calcium carbonate and calcium fluoride, and magnesium phosphate. 



The animal matter called collagen is resolved into gelatin by boiling. 



The earthy and animal constituents of bone are so intimately blended 

 and incorporated the one with the other that it is only by chemical 

 action, as for instance by heat in one case and by the action of acids in 

 another, that they can be separated. Their close union too is further 

 shown by the fact that when by acids the earthy matter is dissolved out, 

 or on the other hand when the animal part is burnt out, the shape of 

 the bone is alike preserved. 



The proportion between these two constituents of bone varies in 

 different bones in the same individual and in the same bone at different 

 ages. 



Structure. -To the naked eye there appear two kinds of structure 



