4'2 HAND-BOOK OF PHYSIOLOGY 



Functions of Cartilage. Cartilage not only represents in the foetus 

 the bones which are to be formed (temporary cartilage), but also offers a 

 firm, but more or less yielding,, framework for certain parts in the de- 

 veloped 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 (Fig. 

 6) ; 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. 



III. BO^E. 



Chemical Composition. Bone is composed of earthy and animal 

 matter 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 

 phosphate, but besides there is a small quantity (about 11 of the 67 per 

 cent.) of calcium carbonate and fluoride, and magnesium phosphate. 



The animal matter 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 dif- 

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



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

 diiferent bones, and in different parts of the same bone, namely, the dense 

 or compact, and the spongy or cancellous tissue. 



Thus, in making a longitudinal section of a long bone, as the humerus 

 or femur, the articular extremities are found capped on their surface by 

 a thin shell of compact bone, while their interior is made up of the 

 spongy or cancellous tissue. The shaft, on the other hand, is formed 

 almost entirely of a thick layer of the compact bone, and this surrounds 



