CHAPTER II 

 THE SKELETAL SYSTEM 



KINDS AND USES OF BONES 



The skeleton may be divided for description into an axial por- 

 tion, consisting of tlie bones of the head, trunk, and tail, and an 

 appendicular portion, which comprises the bones of the limbs. 

 The number of bones in the skeleton of an animal varies with the 

 age, owing to fusion of bones which are separate in the young. 

 Adult horses have 250 bones as a rule. When dried the bones 

 constitute from 7 to 8.5 per cent, of the body weight. The study 

 of the bones is called osteology. 



The skeleton serves as a support or framework for the more 

 delicate and softer parts of the animal. The bones of vertebrates 

 are embedded in the soft structures of the body, and are, there- 

 fore, termed the endoskeleton. Invertebrates, in contradistinc- 

 tion, often have a skeleton placed outside the softer parts, as in the 

 case of the lobster's shell; such an arrangement is called an exo- 

 skeleton. Occasionally a bone is found embedded in a soft organ 

 and does not articulate with the general skeleton; it belongs to the 

 so-called splanchnic skeleton. The os cordis in the ox heart, os 

 penis of the dog, and os rostri in the snout of the hog are examples 

 of bones belonging to the splanchnic skeleton. 



It is customary to divide the bones according to their shape into 

 four classes, as follows: 



1. Long bones, which have a marrow cavity and consist of a 

 shaft and two extremities. They are found exclusively in the 

 limbs and act as columns of support, e. g., femur or thigh bone. 



2. Flat bones, which protect organs and afford extensive surface 

 for the attachment of muscles, e. g., scapula or shoulder-blade. 



3. Short bones, which are cubical in form and occur chiefly in 

 compound joints, e: g., the hock. 



4. Irregular bones, which include all others not coming in the 

 above groups. They are all single and lie along the median 

 plane, e. g., vertebrae. 



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