OSTEOLOGY 



THE SKELETON 

 The term skeleton is applied to the framework of hard structures which sup- 

 ports and protects the soft tissues of animals. In the descriptive anatomy of the 

 higher animals it is usually restricted to the bones and cartilages, although the 

 ligaments which bind these together might well be included. 



In zoology the term is used in a much more comprehensive sense, and includes all the harder 

 supporting: and protecting structures. When the latter are situated externally, they form an 

 exoskeleton, derived from the ectoderm. Examples of this are tlie shells and cliitinous co\'crings 

 of many invertebrates, the scales of fishes, the shields of turtles, and the feathers, hair, and hoofs 

 of the higher vertebrates. The endoskeleton (with which we have to deal at present) is embedded 

 in the soft tissues. It is derived chiefly from the mesoderm, but includes the notochord or primi- 

 tive axial skeleton, which is of entodermal origin. 



The skeleton may be divided primarily into three parts: (1) axial; (2) appen- 

 dicular; (3) splanchnic. 



The axial skeleton comprises the vertebral column, ribs, sternum, and skull. 



The appendicular skeleton includes the bones of the limbs. 



The splanchnic skeleton consists of certain bones developed in the substance 

 of some of the viscera or soft organs, e. g., the os penis of the dog and the os cordis of 

 the ox. 



The number of the bones of the skeleton of an animal varies with age, owing 

 to the fusion during growth of skeletal elements which are separate in the foetus 

 or the young subject. Even in adults of the same species numerical variations 

 occur, e. g., the tarsus of the horse may consist of six or seven bones, and the carpus 

 of seven or eight; in all the domestic mammals the number of coccygeal vertebrae 

 varies considerably. 



The bones are commonly divided into four classes according to their shape 

 and function. 



(1) Long bones (Ossa longa) are typically of elongated cylindrical form with 

 enlarged extremities. They occur in the limbs, where they act as supporting 

 columns and as levers. The cylindrical part, termed the shaft or body (Corpus), 

 is tubular, and incloses the medullary cavity, which contains the medulla or 

 marrow. 



(2) Flat bones (Ossa plana) are expanded in two directions. They furnish 

 sufficient area for the attachment of muscles and afford protection to the organs 

 which they cover. 



(3) Short bones (Ossa brevia), such as those of the carpus and tarsus, present 

 somewliat similar dimensions in length, breadth, and thickness. Their chief func- 

 tion appears to be that of diffusing concussion. Sesamoid bones, which are 

 developed in the capsules of some joints or in tendons, may be included in this 

 group. They diminish friction or change the direction of tendons. 



(4) Irregular bones. This group would include bones of irregular shape, 

 such as the vertebne and the bones of the cranial base; they are median and 

 unpaired. Their functions are various and not so clearly specialized as those of 

 the preceding classes. 



This classification is not entirely satisfactory; some bones, e. g., the ribs, are not clearly 

 provided for, and others might be variously placed. 



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