THE SKELETON. 9 



THE SKELETON. 



The skeleton is that arrangement of bones which 

 supports the soft structures of the body. It consists, in 

 the adult, of 200 bones, as follows : 



, ( Cranium, 8 ) 



Head ] 1 22 



I Face, ... 14 ^ 



Vertebral column, including sacrum, and cocc3 - x, . 26 



Upper extremities, 64 



Ribs, sternum, and os hyoides, . . . .26 

 Lower extremities, . . . . . .62 



200 



The bones are divided into four classes, long, short, 

 flat, and irregular. A long bone presents a shaft, two 

 extremities, and has a medullary cavity. The shaft is 

 cylindrical or prismatic, hollow, filled with yellow mar- 

 row, contained within a delicate investing membrane 

 called the endosteum, which lines the medullary cavity. 

 It is a hollow cylinder of compact tissue, which becomes 

 thinner toward the extremities ; these are expanded and 

 bulky, and Serve for articulation. About the centre of 

 a long bone is the nutrient foramen ; it enters the 

 medullary canal and transmits an artery to the marrow. 

 The direction of the nutrient foramen and canal is clif- 

 erent in different bones : in the long bones of the upper 

 extremities it is directed toward the elbow-joint ; in the 

 femur, toward the hip-joint; and in the bones of the 

 leg, toward the ankles. 



A short bone is thick, irregular, cuboidal, and con- 

 sists of a shell of compact bone inclosing spongy tissue, 

 the alveoli of which are filled with red marrow. The 

 short bones are those of the tarsus and carpus. 



A flat bone is plate-like in form, and consists of two 

 tables of compact bone held together by cancellated 

 tissue. In the bones of the skull the tables of the flat 



