The Articulations. 



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

 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, their adjacent margins 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 

 surfaces are united by tough and elastic fibro-cartilages, as in the joints of the 

 spine, the sacro-iliac, and interpubic articulations ; but in the movable joints, the 

 bones forming the articulation are generally expanded for greater convenience 

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

 capsules of fibrous tissue, called ligaments, and lined by a membrane, the synovial 

 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 synovial 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 evidently designed to form a firm and unyielding support 

 for the articular cartilage. 



The articular will be found described along with the other kinds of cartilage 

 in the Introduction. 



Ligaments are found in nearly all the movable articulations ; they 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. 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 ligaments 

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

 together the adjacent arches of the vertebrae, and the ligamentum nucluo. 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, arranged in the form of a 

 short wide tube, attached by its open ends to the margins of the articular 

 extremities of the bones, and covering the inner surface of the various liga- 

 ments which connect the articulating surfaces. It resembles the serous mem- 

 branes in structure, but differs in the nature of its secretion, which is thick, 

 viscid, and glairy, like the white of egg; and hence termed synovia. The 



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