ARTHROLOGY 



THE ARTICULATIONS OR JOINTS 



An articulation or joint is formed by the union of two or more bones or carti-' 

 lages by other tissue. Bone is the fundamental part of most joints; in some cases 

 a bone and a cartilage, or two cartilages, form a joint. The uniting medium is 

 chiefly fibrous tissue or cartilage, or a mixture of these. Union of parts of the 

 skeleton by muscles (Synsarcosis) , as in the attachment of the thoracic limb in 

 the horse, will not be considered in this section. 



Joints may be classified — (a) anatomically, according to their mode of develop- 

 ment, the nature of the uniting medium, and the form of the joint surfaces; (b) 

 phj'siologically, with regard to the amount and kind of movement or the absence 

 of mobilitjr in them; (c) by a combination of the foregoing considerations. 



The classification of joints is still in a very unsatisfactory state, and unfortunately the same 

 term is used in various senses by different authors. The two main subdi\'isions proposed by Hep- 

 burn are: (1) Those in which the uniting medium is coextensive with the opposed joint surfaces, 

 and in which a direct union of these surfaces is thereby effected. (2) Those in which the uniting 

 medium has undergone interruption in its structural continuity, and in which a cavity of greater 

 or less extent is thus formed in the interior of the joint. This distinction is of considerable im- 

 portance chnically. 



Three chief subdivisions of joints are usually recognized — viz., synarthroses, 

 diarthroses, and amphiarthroses. 



SYNARTHROSES 



In this group the segments are united by fibrous tissue or cartilage, or a mix- 

 ture of the two in such a manner as practically to preclude movement; hence they 

 are often termed fixed or immovable joints. There is no joint cavity. Most of 

 these joints are temporary, the uniting medium being invaded by the process of 

 ossification, with a resulting ankylosis or synostosis. The chief classes in this 

 group of joints are as follows: 



(1) Suture. — This term (Sutura) is applied to those joints in the skull in which 

 the adjacent bones are closely united by fibrous tissue — the sutural ligament. In 

 many cases the edges of the bones have irregular interlocking margins, forming the 

 sutura serrata, e. g., the frontal suture. In others the edges are beveled and 

 overlap, forming the sutura squamosa, e. g., the parieto-temporal suture. If the 

 edges are plane or slightly roughened, the term sutura harmonia is applied to the 

 joint, e. g., the nasal suture. 



(2) Syndesmosis. — In these the uniting medium is white fibrous or elastic 

 tissue or a mixture. As examples are the union of the shafts of the metacarpal bones 

 and the attachments to each other of costal cartilages. 



(3) Synchondrosis. — In these the two bones are united by cartilage, e. g., 

 the joint between the basilar part of the occipital bone and the sphenoid bone. 

 Very few of these joints are permanent. 



(4) Symphysis.— This term is usually limited to a few median joints which 

 connect symmetrical parts of the skeleton, e. g., symphysis pelvis, symphysis 

 mandibulse. The uniting medium is cartilage and fibrous tissue. In some cases 

 a cleft-like rudimentary joint cavity occurs. 



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