APPARATUS OF MOTION IN GENEBAL. 131 



are composed of a compact tissue externally, and a cellular 

 and reticulated or cancellous internally. 



In the interior of the long bones are found cavities con- 

 taining marrow. The tissue itself, when examined under the 

 microscope, seems composed of tubes or cellules, surrounded 

 with concentric lamellae, between which may be seen opaque 

 ovoid corpuscles. 



262. The bones vary much in form, and anatomists 

 have divided them into long, short, and broad. Their sur- 

 faces present elevations (processes) and depressions, some of 

 which are for the attachment of muscles. 



263. Articulation of the Bones. By articulation is 

 meant the union of two or more bones ; they are divided into 

 the immovable or fixed, and the moveable. The fixed may be 

 effected by juxtaposition, suture, and implantation; of the 

 first, certain bones of the face offer an example ; the bones of 

 the cranium unite chiefly by suture, and the teeth are fixed 

 into the alveoli by implantation ; this mode of articulation is 

 also called gomphosis. 



264. In the moveable articulations, the bones are held 

 in their place by ligaments maintaining them in juxtaposition. 

 This bond of union is sometimes a fibro-cartilage, which 

 permits the bones to move simply by reason of its elasticity ; 

 in other joints, the bones slide over each other, and are held 

 in their place by ligaments attached to both bones, and per- 

 mitting of motion only in certain directions ; this mode of 

 articulation is called articulation Ijy contiguity, and it 

 always exists where extensive motions take place. To meet 

 the friction caused by this, the extremities of the bones are 

 further provided with a cartilage of incrustation, and a syno- 

 vial membrane representing a serous membrane in minia- 

 ture, and is constantly bedewed with an unctuous fluid, called 

 synovia, serving the purpose of a joint-oil. By means of this 

 membrane the air is effectually excluded from the joints, so 

 that the relative position of the bones to each other is further 

 secured by the pressure of the external air on the exterior of 

 the limb. 



265. Action of the Muscles on the Bones. All the 

 muscles destined to perform extensive movements are attached 

 to the bones by their two extremities, so that by contracting 

 they displace the more moveable bone in the direction of that 

 which is less moveable or fixed. Thus, generally speaking, 

 the muscles intended to move the fingers proceed from the t 

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