OSTEOLOGY. 



general framework of the body is built up mainly of a series of bones, 

 - supplemented, however, in certain regions by pieces of cartilage; the bony 

 part of the framework constitutes the skeleton. 



In the skeleton of the adult there are 206 distinct bones, as follows : 



Axial 



Skeleton 



Appendicular 

 Skeleton 



Vertebral column 

 Skull .... 

 Hyoid bone . 

 Ribs and sternum 



f Upper extremities 

 \ Lower extremities 



Auditory ossicles 



Total 



26 



22 



1 



25 



74 

 64 

 62 



126 

 6 



206 



The patellae are included in this enumeration, but the smaller sesamoid bones 

 are not reckoned. 



Bones are divisible into four classes: Long, Short, Flat, and Irregular. 



Long Bones. The long bones are found in the limbs, and each consists of a body 

 or shaft and two extremities. The body, or diaphysis is cylindrical, with a central 

 cavity termed the medullary canal; the wall consists of dense, compact tissue 

 of considerable thickness in the middle part of the body, but becoming thinner 

 toward the extremities; within the medullary canal is some cancellous tissue, 

 scanty in the middle of the body but greater in amount toward the ends. The 

 extremities are generally expanded, for the purposes of articulation and to afford 

 broad surfaces for muscular attachment. They are usually developed from sep- 

 arate centers of ossification termed epiphyses, and consist of cancellous tissue 

 surrounded by thin compact bone. The medullary canal and the spaces in the 

 cancellous tissue are filled with marrow. The long bones are not straight, but 

 curved, the curve generally taking place in two planes, thus affording greater 

 strength to the bone. The bones belonging to this class are: the clavicle, humerus, 

 radius, ulna, femur, tibia, fibula, metacarpals, metatarsals, and phalanges. 



Short Bones. Where a part of the skeleton is intended for strength and com- 

 pactness combined with limited movement, it is constructed of a number of short 

 bones, as in the carpus and tarsus. These consist of cancellous tissue covered 

 by a thin crust of compact substance. The patellae, together with the other 

 sesamoid bones, are by some regarded as short bones. 



Flat Bones. Where the principal requirement is either extensive protection or 

 the provision of broad surfaces for muscular attachment, the bones are expanded 

 into broad, flat plates, as in the skull and the scapula. These bones are composed 

 of two thin layers of compact tissue enclosing between them a variable quantity 

 of cancellous tissue. In the cranial bones, the layers of compact tissue are famili- 

 arly known as the tables of the skull; the outer one is thick and tough; the inner 

 is thin, dense, and brittle, and hence is termed the vitreous table. The intervening 



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