STRUCTURE OF JOINTS. 183 



doubts tins. Lymphatic vessels and nerves have not, as yet, been traced in its 

 substance. 



Fibro-cartilage is also employed in the construction of the joints, contributing 

 to their strength and elasticity. It consists of a mixture of white fibrous and 

 cartilaginous tissues in various proportions ; it is to the first of these two consti- 

 tuents that its flexibility and toughness is chiefly owing, and to the latter its 

 elasticity. The fibro- cartilages admit of arrangement into four groups, inter- 

 articular, connecting, circumferential, and stratiform. 



The interarticular fibro-cartilages or menisci are flattened fibro-cartilaginous 

 plates, of a round, oval, or sickle-like form, interposed between the articular car- 

 tilages of certain joints. They are free on both surfaces, thinner toward their 

 centre than at their circumference, and held in position by their extremities being 

 connected to the surrounding ligaments. The sy no vial membrane of the joint is 

 prolonged over them a short distance from their attached margin. They are 

 found in the temporo-maxillary, sterno-clavicular, acromio-clavicular, wrist and 

 knee-joints. These cartilages are usually found in those joints most exposed to 

 violent concussions, and subject to frequent movement. Their use is, to maintain 

 the apposition of the opposed surfaces in their various motions ; to increase the 

 depth of the articular surface, and give ease to the gliding movement ; to mode- 

 rate the effects of great pressure, and deaden the intensity of the shocks to which 

 they may be submitted. 



The connecting fibro-cartilages are interposed between the bony surfaces of those 

 joints which admit of only slight mobility, as between the bodies of the vertebrae, 

 and the symphysis of the pubes ; they exist in the form of disks, intimately adhe- 

 rent to the opposed surfaces, being composed of concentric rings of fibrous tissue, 

 with cartilaginous laminae interposed, the former tissue predominating towards the 

 circumference, the latter towards the centre. 



The circumferential fibro-cartilages consist of a rim of fibro-cartilage, which 

 surrounds the margin of some of the articular cavities, as the cotyloid cavity of 

 the hip, and the glenoid cavity of the shoulder ; they serve to deepen the articular 

 surface and protect the edges of the bone. 



The stratiform fibro-cartilages are those which form a thin layer in the osseous 

 grooves, through which the tendons of certain muscles glide. 



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, admirably 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 nuchse. '*- ^ 



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 ex- 

 tremities of the bones, and covering the inner surface of the various ligaments 

 which connect the articulating surfaces. It resembles the serous membranes in 

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

 viscid, and glairy, like the white of egg, and 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 

 of the cartilages ; but in the adult it is wanting, excepting at their circumference, 

 upon which it encroaches for a short distance : it then invests the inner surface of 

 the capsular or other ligaments inclosing the joint, and is reflected over the 

 surface of any tendons passing through its cavity, as the tendon of thaPopliteus 



