no 



HUMAN ANATOMY. 



the articular surface of a certain bone may be plane, convex, or concave in different 



persons. 



The capsule. — Every joint, with possibly some exceptions in the carpus and 

 the tarsus, is enclosed by a capsule,^ or capsular ligame7it, which arises from the peri- 

 osteum near the borders of the articular cartilage and surrounds the joint. This 

 envelope consists of a membrane, often containing fat within its meshes, composed 

 of two layers, the inner delicate synovial membrane and the external fibrous layer. 

 The latter, while in some places very thin, is usually strengthened by the incorpora- 

 tion of fibrous bands which, from their position, are known as lateral, anterior, or 

 posterior ligaments. These bands are of strong, non-elastic fibrous tissue which 

 under ordinary circumstances do not admit of stretching. The strength and 

 security of the joint are often materially increased through thickenings of fasciae and 

 expansion of tendons which blend with the underlying capsule. The capsule must 

 be large enough to allow the characteristic movements of the joint ; consequently, 

 when the bone is moved in any particular direction that side of the capsule is relaxed 

 and thrown into folds. These folds are drawn out of the way either by small special 

 muscles situated beneath those causing the chief movement or by fibres from the 



deeper surfaces of these latter muscles. In 

 the joints of the arches of the vertebrae, there 

 being no muscles inside the spinal canal, a dif- 

 ferent arrangement exists for the inner side c: 

 the capsule, elastic tissue there taking the 

 place of muscle. The relation of the insertion 

 of the capsule to the line of the epiphysis is 

 important. Although this point is fully con- 

 sidered in the description of the individual 

 joints, it may be here stated that, as a rule, in 

 the long bones, the capsule arises very near 

 the line of the epiphysis. 



The synovial membrane which lines 

 the interior of the capsule and other portions 

 of the joint, except the surfaces of the articular 

 cartilages, consists of a delicate connective- 

 tissue sheet, containing many branched and 

 flattened connective-tissue cells. The latter, 

 where numerous, as is the case except at points 

 subjected to considerable pressure, are ar- 

 ranged on the free surface of the synovial mem- 

 brane as a more or less continuous layer, often 

 spoken of as the endothelium of the synovial 

 sac. Since in many places the layer of connective-tissue elements is imperfect and 

 the component cells retain their stellate form, the cellular investment of the joint- 

 cavity is at best endothelioid, suggesting, rather than constituting, an endothelium. 

 The synovial membrane is in certain places pushed inward by accumulations of fat 

 of definite shape between it and the capsule. It is also prolonged, as the synovial 

 fringes,"^ into any space that might otherwise be left vacant in the various movements. 

 They are alternately drawn in or thrust out, according to circumstances. Some- 

 times pieces of them, or of fibro- cartilage, become detached in the joint, giving rise 

 to much trouble. 



The cavity which is found when a joint is opened on the cadaver, with the 

 tissues dead and relaxed, easily suggests a false impression. It is to be remembered 

 that the synovial fluid normally is present in quantity little more than sufficient to 

 lubricate the joint, and that in life all the parts are strongly pressed together so that 

 no true cavity exists. This is well shown by frozen sections. 



Certain so-called intra-articular ligaments, as the ligamentum teres of the hip, 

 or the crucial ligaments of the knee-joint, are found in the adult, roughly speaking, 

 inside the joint. The sketch of development given above shows that they cannot 

 be truly within the ar1;icular cavity. In fact, either they wander in from the capsule, 

 carrying with them a reflection of synovial membrane, or they are the remnants of 



' CaDsala articularis. " Plicae s"noviaIes. 



Capsu) 



Synovial membrane 



Articular cartilage 



Joint-cavity 



Reflection of syno- 

 vial membrane. 



Epiphyseal bone. 



Diagram showing the parts of a typical joint. 



