44 THE HUMAN BODY 



interweaving is to give the cartilage a yellow color and a high de- 

 gree of elasticity. Cartilage of this sort is found in the external 

 ear, the epiglottis, and in certain parts of the larynx. 



Fibrocartilage is really a dense fibrous connective tissue within 

 whose spaces a certain amount of matrix material has been de- 

 posited. It makes up the intervertebral disks, pads which are 

 interposed between the bones of the vertebral column, and is 

 found also in certain joints, notably the knee-joints and the ar- 

 ticulations of the lower jaw. 



Bone. The bones which make up the skeleton vary greatly in 

 shape and size, ranging from the long cylindrical bones of the 

 arm and leg to the flat skull bones, and the tiny irregularly shaped 

 ossicles of the middle ear. They all, however, have a similar 

 microscopic structure and similar chemical composition. 



The bones may be classified according to their origin as mem- 

 brane bones or cartilage bones. To the first group belong the flat 

 bones of the skull and the bones of the face (see p. 56) . They do 

 not replace cartilage but develop upon a foundation of connective 

 tissue. The so-called cartilage bones replace the temporary 

 cartilages and make up the whole of the bony skeleton, except the 

 membrane bones mentioned above. 



The Process of Bone Formation is complicated, and can be 

 described only very briefly here. At the beginning of the develop- 

 ment of a membrane bone the strands of connective tissue upon 

 which the bone is to be built become covered with peculiar small 

 cells which are bone-producing cells or osteoblasts. These osteo- 

 blasts deposit upon the strands whereon they rest albuminoid 

 material which constitutes the organic matrix of bone. There is 

 thus produced a rather open network of bone matrix. By the 

 deposition within the matrix of lime-salts it takes on the character 

 of true bone. The original connective tissue is thus replaced by a 

 network of bony spicules. 



The surfaces of this bony mass now become covered with a stout 

 connective tissue membrane, the periosteum, whose inner surface 

 is beset with osteoblasts. These deposit upon the underlying mass 

 a layer of compact bone. Thus the fully formed membrane bone 

 consists of outer surfaces of compact bone inclosing a mass of 

 spongy bone. 



The replacement of temporary cartilage by bone proceeds from 



