THE ARTICULATIONS. 



fTlHE various bones of which the Skeleton consists are connected together at 

 J_ different parts of their surfaces, and such a connection is designated by the 

 name of Joint or Articulation. If the joint is immovable, as between the cranial and 

 most of the facial bones, the adjacent margins of the bones are applied in almost 

 close contact, a thin layer of fibrous membrane, the sutural ligament, and, at the 

 base of the skull, in certain situations, a thin layer of cartilage, being interposed. 

 Where slight movement is required, combined with great strength, the osseous sur- 

 faces are united by tough and elastic fibro-cartilages, as in the joints between the 

 bodies of the vertebrae and interpubic articulations ; but in the movable joints the 

 bones forming the articulation are generally expanded at the ends for greater con- 

 venience of mutual connection, covered by cartilage, held together by strong bands 

 or capsules of fibrous tissue called ligaments, and partially lined by a membrane, 

 the synovia!, membrane, which secretes a fluid to lubricate the various parts of 

 which the joint is formed; so that the structures which enter into the formation 

 of a joint are bone, cartilage, fibro-cartilage, ligament, and synovia! membrane. 



Bone constitutes the fundamental element of all the joints. In the long bones 

 the extremities are the parts which form the articulations ; they are generally 

 somewhat enlarged, consisting of spongy cancellous tissue, with a thin coating of 

 compact substance. In the flat bones the articulations usually take place at the 

 edges, and, in the short bones at various parts of their surface. The layer of 

 compact bone which forms the articular surface, and to which the cartilage is 

 attached, is called the articular lamella. It is of a white color, extremely dense, 

 and varies in thickness. Its structure differs from ordinary bone-tissue in this 

 respect, that it contains no Haversian canals, and its lacunae are much larger than 

 in ordinary bone and have no canaliculi. The vessels of the cancellous tissue, as 

 they approach the articular lamella, turn back in loops, and do not perforate it ; 

 this layer is consequently more dense and firmer than ordinary bone, and is evi- 

 dently designed to form a firm and unyielding support for the articular cartilage. 



The cartilage, which covers the articular surfaces of bone, and is called the 

 articular, will be found described, with the other varieties of cartilage, in the section 

 on General Anatomy (page 51). 



Ligaments consist of bands of various forms, serving to connect together the 

 articular extremities of bones, and composed mainly of bundles of white fibrous 

 tissue placed parallel with, or closely interlaced with, one another, and presenting 

 a white, shining, silvery aspect. A ligament is pliant and flexible, so as to allow 

 of the most perfect freedom of movement, but strong, tough, and inextensile, so 

 as not readily to yield under the most severely applied force ; it is consequently 

 well adapted to serve as the connecting medium between the bones. Some liga- 

 ments consist entirely of yellow elastic tissue, as the ligamenta subflava, which 

 connect together the adjacent arches of the vertebrae and the ligamentum nuchae 

 in the lower animals. In these cases it will be observed that the elasticity of the 

 ligament is intended to act as a substitute for muscular power. 



Synovial membrane is a thin, delicate membrane of connective tissue, with 

 branched connective-tissue corpuscles. Its secretion is thick, viscid, and glairy, 

 like the white of egg, and is hence termed synovia. The synovial membranes 

 found in the body admit of subdivision into three kinds articular, bursal, and 

 vaginal. 



The articular synovial membranes are found in all the freely movable joints. 

 In the foetus this membrane is said, by Toynbee, to be continued over the surface 



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