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CHAPTER VI. 



SVNOVIAL MEMBRANES. SEROUS MEMBRANES. VARIETIES OF JOINTS 



MECHANISM OF THE SKELETON. 



THE different forms of bones, when united according to various 

 mechanical contrivances, constitute the skeleton. The framework 

 of the body, being thus formed of several pieces jointed together, is 

 admirably arranged for extended, or for minute and nicely adjusted 

 motions, and for distributing concussions over a large surface. The 

 interposition of discs, or laminae, of elastic cartilage, or fibro-carti- 

 lage, between some bones, contributes to the latter object, by inter- 

 rupting the medium through which the shock would be conducted, 

 as well as by the elasticity of the intervening substances ; and, at 

 the same time, these discs, by their intimate adhesion to the opposed 

 osseous surfaces, serve as powerful bonds of union between them. 

 Joints of this kind (synar thro dial) enjoy a very limited degree of 

 motion, which is entirely due to the yielding and elastic nature of 

 the interposed material. When a greater range of motion is re- 

 quired than can be obtained in this way, the surfaces of the osseous 

 segments are constructed so as to glide the one upon the other in 

 certain directions, which are determined by the form of the articular 

 surfaces, and by the positions at which the connecting ligaments are 

 placed. Here the bond of union consists of the ligaments and 

 the surrounding muscles ; the osseous segments are not, as in the 

 former instance, continuous with each other through the inter- 

 posed texture, but are separated by a space which is called the 

 cavity of the joint. Each osseous surface is encrusted by a layer of 

 articular cartilage adapted to its form, and the cavity of the joint 

 is lined by a delicate membrane, which secretes a peculiar viscid 

 matter, synovia, admirably suited to lubricate the surface. This 

 membrane is termed synovial , and is constantly present in the diar- 

 throdial joints. 



The articular synovial membrane forms a closed bag, placed 



between the articular surfaces of the bones. Its free surface is 



smooth and moist ; its attached surface adheres by very fine areolar 



, tissue to the ligaments of the joint, and to the cartilages encrusting 



