vn STRUCTURE OF A BONE 305 



very dense and compact osseous matter, but often contain- 

 ing a great central cavity (b) which is filled with a very 

 delicate vascular and fibrous tissue loaded with fat called 

 marrcw. Towards the two ends of the bone, the 

 compact matter of the shaft thins out, and is replaced by 

 a much thicker but looser sponge-woi'k of bony plates and 

 fibres, which is termed the cancellous tissue of the 

 bone. The surface even of this part, however, is still 

 formed by a thin sheet of denser bone. 



At least one end of each of these bony levers is fashioned 

 into a smooth, articular surface, covered with cartilage, 

 which enables the relatively fixed end of the bone to play 

 upon the corresponding surface of some other bone with 

 which it is said to be articulated (see p. 319), or, contrari- 

 wise, allows that other bone to move upon it. 



It is one or other of these extremities which plays the 

 part of fulcrum when the bone is in use as a lever. 



Thus, in the accompanying figure (Fig. 92) of the bones 

 of the upper extremity, with the attachments of the biceps 

 muscle to the shoulder-blade and to one of the two bones 

 of the fore-arm called the radius, P indicates the point of 

 action of the power (the contracting muscle) upon the 

 radius. 



It usually happens that the bone to which one end of a 

 muscle is attached is absolutely or relatively stationary ; 

 while that to which the other is fixed is movable. In 

 this case, the atta,chment to the stationary bone is termed 

 the origin, that to the movable bone the insertion, of 

 the muscle. 



The fibres of muscles are sometimes fixed directly into 

 the parts which serve as their origins and insertions ; but, 

 more commonly, strong cords or bands of fibrous tissue, 

 called tendons, are interposed between the muscle 

 proper and its place of origin or insertion. When the 

 tendons play over hard surfaces, it is usual for them to be 

 separated from these surfaces by sacs containing fluid, 

 which are called bursse ; or even to be invested by 



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