46 ELEMENTARY TISSUES. 



posticus, iii the sole of the foot, and usually in the neighboring 

 tendon of the peroneus longus. 



The uses of cartilage are the following : in the joints, to 

 form smooth surfaces ibr easy friction, and to act as a buffer, 

 in shocks ; to bind bones together, yet to allow a certain degree 

 of movement, as between the vertebrae ; to form a firm frame- 

 work 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 pro- 

 portion of about 67 per cent, of the former to 33 per cent, of 

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

 phate of lime, but besides there is a small quantity, about 11 

 of the 67 per cent., of carbonate of lime, with minute quantities 

 of some other salts. The animal matter is resolved into gela- 

 tin 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 dis- 

 solved out, or, on the other hand, 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 hume- 

 rus or femur, the articular extremities are found capped on 

 their surface by a thin shell of compact bone, while their in- 

 terior 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 (p. 42). 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. The spaces in the cancellous tissue are 

 filled by a species of marrow, which differs considerably from 



