20 MAMMALIAN AXATOMY 



common name is used as a technical name ; for example, rib is almost 

 universally used instead of the Latin cox fa. 



CLASSIFICATION OF BONES. 



Bones are often grouped, according to their shape, into four classes : 

 long bones, short bones, flat or broad bones, and irregular bones. 

 Although this classification is vague and of little scientific value, 

 there is sufficient difference in the character of the bony structure in 

 the groups to render some such classification convenient, vague though 

 it be. 



Long bones have one of their diameters conspicuously greater than 

 the others. They possess a central shaft, or diaphysis, and two ends, or 

 epiphyses. The interior of the shaft is a medullary cavity filled with 

 marrow. The long bones are the bones of the limbs, forming levers 

 whereon the muscles act. 



Short bones have no one diameter greatly exceeding the others. 

 They are not divided into diaphysis and epiphyses, and they have 110 

 medullary cavity. Short bones are found in the carpus, the tarsus, 

 and the sternum, places where strength and elasticity with slight 

 movement are required. 



Flat bones have one diameter conspicuously less than the others. 

 In structure they consist of two plates of compact bony tissue separated 

 by a thin layer of cancellous tissue. The flat bones form the walls of 

 cavities or furnish large areas for muscular attachment. Such are the 

 bones of the cranial vault, the innominate bones, and the scapula?. 



Irregular bones are the remaining bones which do not fall natu- 

 rally in any one of the other classes. Their shape is irregular, and 

 in structure they are like both the short bones and the flat bones. 

 Irregular bones are found in the skull, and as vertebra 1 in the vertebral 



column. 



UNION OF BONES. 



The consideration of the manner in which bones are joined together 

 properly belongs to Syndesmology, which treats of articulations. In 

 order to understand the bones themselves, however, some general facts 

 concerning their modes of union must be made clear. 



Bones are united firmly and immovably, or are movable and 

 capable of different degrees of motion. All varieties of union may be 

 grouped under two heads : 



