CHAPTER XIII 



THE SKELETAL SYSTEM 



The skeletal system consists of a series of bones and cartilages 

 which are united by special structures to form the supporting frame- 

 work of the body. Under this head are considered: (i) bones, (2) 

 marrow, (3) cartilages, (4) articulations. 



The Bones 



A bone considered as an organ consists of bone tissue laid down 

 in a definite and regular manner. If a longitudinal section be made 

 through the head and shaft of a long bone, the head of the bone and 

 also part of the shaft are seen to be composed of anastomosing bony 



Fig. 115. — Section of Spongy Bone. X75. (Technic 3, p. 202.) a, Marrow space ;'6, 

 group of fat cells; c, blood-vessel; d, trabeculae of bone. 



trabeculas enclosing cavities. This is known as cancellous or spongy 

 hone. The shaft of the bone consists of a large central cavity sur- 

 rounded by spongy bone, which, however, passes over on its outer 

 side into a layer of bone of great density and known as hard or compact 

 hone. Spongy bone forms the ends and Hnes the marrow cavities of 

 the long bones, and occurs also in the interior of short bones and flat 



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