44 HANDBOOK OF PHYSIOLOGY. 



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. 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 percent) 

 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 ac- 

 tion, as, for ins-tance, 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 

 ages. 



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

 different 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 

 a central canal, the medullary cavity so called from its containing the 

 medulla or marrow. 



In the flat bones, as the parietal bone or the scapula, one layer of the 

 cancellous structure lies between two layers of the compact tissue, and 

 in the short and irregular bones, as those of the carpus and tarsus, the 

 cancellous tissue alone fills the interior, while a thin shell of compact 

 bone forms the outside. 



Marrow. There are two distinct varieties of marrow the red and 

 yellow. 



