CHAP. VI.] THE JOINTS. 131 



the adhesive inflammation as the serous are, which seems more to 

 be accounted for by the nature of their secretion, than by any dif- 

 ference of their structure. The proneness of these membranes to 

 the effusion of coagulable lymph seems to be due to the extreme 

 tenuity of the layer of epithelium which separates the nutrient 

 blood-vessels from the cavity of the serous membrane. The lymph 

 effused becomes gradually converted into areolar tissue and vessels, 

 usually constituting what are termed adhesions, but sometimes 

 forming merely a thickened condition of the membrane. 



Of the Joints. A joint, or articulation, may be denned to be the 

 union of any two segments of an animal body, through the inter- 

 vention of a structure or structures different from both. 



The most perfect and elaborate forms of the articulations are 

 those which are seen in animals that possess a fully developed in- 

 ternal skeleton, and in none may they be studied with more advan- 

 tage than in man. In the human subject, and in the vertebrated 

 animals generally, we have, indeed, particular occasion to admire 

 the articulations as " mirabiles commissuras, et ad stabilitatem aptas 

 et ad artus finiendos accommodatas, et ad motum, et ad omnem 

 corporis actionem/' 



The textures which form the joints, are bone, cartilage, fibro- 

 cartilage, ligaments, synovial membrane. Bone constitutes the 

 fundamental part of all joints ; ligament variously modified is em- 

 ployed in all as a bond of union ; but the three remaining textures 

 are present chiefly in those joints which enjoy a free gliding 

 motion. 



In addition to the structures already named as entering intrinsi- 

 cally into the formation of joints, we find that the tendons and 

 muscles, which lie in the immediate vicinity of or which surround 

 the joints, contribute much to their strength and security. In 

 joints of the hinge kind we generally see the anterior and posterior 

 parts protected more or less by the tendons of muscles, and even by 

 muscles themselves passing from one segment of a limb to another; 

 and here it frequently happens, that the tendon is bound down on 

 the bones which form the member, by a fibrous expansion of great 

 strength, lined by a synovial membrane of the same characters as 

 the articular, but adapted in its form to the osseo-fibrous canal in 

 which the tendon is placed, e.g. the tendons of the fingers. The 

 protection and strength afforded by muscles is particularly evinced 

 in the case of the shoulder- joint, where the capsular ligament is 

 closely embraced by four muscles, whose tendons become identified 

 with the fibrous capsule as they go to be inserted into the bone. 



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