BOXE. 45 



form a firm framework and protection, yet without undue 

 stiffness or weight, as in the larynx and chest walls ; to 

 deepen joint-cavities, as in the acetabulum, yet not so as 

 to restrict the movements of the bones ; to be, where such 

 qualities are required, firm, tough, flexible, elastic, and 

 strong. 



Structure of Bones and Teeth. 



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 phosphate of lime, but besides there is a small quantity, 

 about 1 1 of the 67 per cent., of carbonate of lime, with 

 minute quantities of some other salts. The animal matter 

 is resolved into gelatine 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 sepa- 

 rated. Their close union, too, is further shown by the 

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

 or, on other band, when the animal part is burnt out, 

 the general shape of the bone is alike preserved. 



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 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 contact 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 sur- 

 rounds a central canal, the medullary cavity so called from 

 its containing the medulla or marrow (p. 39). 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 



