CLASSES OF BONES 55 



levers which permit the power of locomotion and 

 prehension. Long bones possess a shaft and an upper 

 and lower extremity. The shaft or the diaphysis is 

 cylindrical, containing in the centre a hollow cavity, 

 the medullary canal, in which during life is the bone- 

 marrow. Surrounding this is dense compact tissue of 

 considerable thickness, but not so thick near the 

 extremities. The extremities or epiphyses are usually 

 expanded to enable one bone to articulate with another 

 and to afford attachment of muscles. They are usually 

 developed from separate centres of ossification called 

 epiphyses. Examples of long bones are the femur or 

 thigh bone, the humerus or bone of the arm, the clavicle 

 or collar bone, the radius and ulna (bones of the fore- 

 arm), the tibia and fibula (bones of the leg), the meta- 

 carpal, metatarsal, and phalanges (the small bones of 

 the hand and foot) respectively. 



Short Bones. These are found in that portion of the 

 body where strength and compactness are required 

 and the motion of the part is limited, as the bones of 

 the carpus (wrist) and tarsus (instep). These bones 

 consist of cancellous tissue covered by compact bone. 

 They are held firmly together by ligaments. Some 

 include the patellae and sesamoid bones under this 

 variety. 



Flat Bones. These are found wherever protection 

 is required, or a broad surface for the attachment 

 of muscles is essential, as the bones of the skull and 

 the scapulae (shoulder-blade) . Flat bones are composed 

 of two thin layers of compact tissue, between which is 

 a variable amount of cancellous tissue. In the bones 

 of the skull these layers of compact tissue are termed 

 the tables of the skull ; the innermost is thin and called 

 the vitreous table. The cancellous tissue between the 

 layers is called the diploe. 



The flat bones are the occipital, parietal, frontal, nasal, 

 lacrymal, vomer, scapula, sternum, ribs, and some authors 

 include the patella. 



