I * The Skeletal System. 99 



adult structure is completed. Finally when this occurs, the zone of 

 cartilage is completely ossified, the division is closed, and no further 

 growth occurs. 



Membrane Bones. — Most bones of the mammalian skeleton are 

 of the cartilage type just described. However, a few of the bones of 

 the skull, jaws, and shoulder girdle are of the membrane type. Mem- 

 brane bones are formed in the skin tissues and are derived from modi- 

 fied scales. 



These bones develop as thin bony plates in the dermis. Gradually 

 they grow in size and become thicker by the deposition of bone on 

 the outer and inner surfaces. These bony plates then sink in to become 

 part of the skeleton. 



DIVISIONS OF THE MAMMALIAN SKELETON 



Some 206 different bones make up the human skeleton. These are 

 of many shapes and sizes, depending upon their location and the 

 functions which they perform. These bones may be conveniently 

 grouped into the various divisions of the skeleton in the following 

 manner. 



A. Axial skeleton 



Skull 

 Vertebrae 

 Ribs 

 Sternum 



B. Visceral 



Portions of the jaw 



Hyoid apparatus 



Cartilages of the larynx, epiglottis, and trachea 



C. Appendicular 



Pectoral girdle and forelimbs 

 Pelvic girdle and hindlimbs 



D. Miscellaneous bones. 



Bones have many protuberances, depressions, and passageways. 

 Various terms have been used to describe these dififerent features. 

 Before beginning a consideration of the individual bones, it will be 

 best to mention soine of the principal terms used. 



Condyle: A smooth protuberance at the end of a bone which articu- 



lates with another bone. 



