SKELETON. 133 



THE SKELETON. 



The skeletal structures of the vertebrates may be either 

 membranous, cartilaginous, or bony (osseous) in character ; and 

 in development certain portions may pass successively through 

 all of these phases in attaining the adult condition ; or the 

 cartilage stage may be skipped, the membrane developing 

 directly into bone ; or again, the cartilaginous condition may 

 be the final stage of the skeleton. 



The membranous skeleton consists of connective tissue 

 cells, and in its highest development forms sheets or masses 

 of fibrous tissue. From it cartilage is developed by a great 

 increase in the number of cells, the tissue in what has been 

 called the procartilage stage consisting of closely compacted 

 polygonal cells with large nuclei. These cells rapidly secrete 

 an intercellular substance (chondrin), and thus the tissue be- 

 comes converted into cartilage, the extent and solidity of which 

 are dependent upon the amount of this matrix. In the conver- 

 sion of cartilage into bone this matrix is dissolved ; and around 

 the margins of the cavities thus produced bone-forming cells 

 (osteoblasts) arrange themselves, and these, secreting lime salts 

 (carbonate and phosphate) around themselves, gradually build 

 up the bone. In the lower vertebrates this process begins 

 upon the outside of the cartilages and proceeds toward the 

 interior ; but in the higher forms, besides this perichondrial 

 ossification, centres of ossification appear within the cartilage, 

 and from these the ossification extends peripherally. In the 

 conversion of membrane into bone there is the same appearance 

 of osteoblasts in and upon the tissue as described above, and 

 these produce the bony substance in the same way. The result 

 in either case is the same, and it is not possible by histological 

 means to distinguish between cartilage bones and membrane 

 bones ; this depends entirely upon development. As will appear 

 later, the distinction between the two is very important. 



Increase in the size of membranes and cartilage is accom- 

 plished by additions to the exterior as well as by increase in 

 the interstitial substance. In the case of bone this interstitial 



