THE CARTILAGE. 



1095 



margins and extremities to the surrounding ligaments. The synovial membrane 

 of the joint is prolonged over them a short distance from their attached margins. 

 They are found in the temporo-mandibular, sterno-clavicular, acromio-clavicular, 

 wrist- and knee-joints. These cartilages are usually found in those joints which are 

 most exposed to violent concussion 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 surfaces and give ease to the gliding movement ; 

 to moderate the effects of great pressure and deaden the intensity of the shocks to 

 which the parts may be subjected. Humphry has pointed out that these inter- 

 articular fibro-cartilages serve an important purpose in increasing the variety of 

 movements in a joint. Thus, in the knee-joint there are two kinds of motion, 

 viz., angular movement and rotation, although it is a hinge joint, in which, as 

 a rule, only one variety of motion is permitted ; the former movement takes place 

 between the condyles of the femur and the interarticular cartilage, the latter between 

 the cartilage and the head of the tibia. So, also, in the temporo-mandibular joint, 

 the upward and downward movement of opening and shutting the mouth takes place 

 between the fibro-cartilage and the jaw-bone, the grinding movement between the 

 glenoid cavity and the fibro-cartilage, the latter moving with the jaw-bone. 



2. 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 between the pubic bones. They form disks, which adhere closely to 

 both of the opposed surfaces, and are composed of concentric rings of fibrous tissue, 

 with cartilaginous laminae interposed, the former tissue predominating toward the 

 circumference, the latter toward the centre. 



3. 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 to protect its edges. 



4. The stratiform fibro-cartilages are those which form a thin coating to osseous 

 grooves through which the tendons of certain muscles glide. Small masses of fibro- 

 cartilages are developed also in the tendons of some muscles, where they glide over 

 bones, as in the tendons of the Peroneus longus and the Tibialis posticus. 



Yellow or elastic fibro-cartilage is found in the human body in the auricle of 

 the external ear, the Eustachian tubes, the cornicula laryngis, and the epiglottis. 

 It consists of cartilage-cells and a matrix, the latter being pervaded in every direc- 

 tion, except immediately 

 around each cell, where there 

 is a variable amount of non- 

 fibrillated hyaline, intercellu- 

 lar substance, by a network 

 of yellow elastic fibres, 

 branching and anastomosing 

 in all directions (Fig. 623). 

 The fibres resemble those of 

 yellow elastic tissue, both in 

 appearance and in being un- 

 affected by acetic acid ; and 

 according to Rollett their con- 

 tinuity with the elastic fibres 

 of the neighboring tissue ad- 

 mits of being demonstrated. 



The distinguishing feature 

 of cartilage as to its chemical composition is that it yields on boiling a substance 

 called chondrin, very similar to gelatin, but differing from it in several of its reac- 

 tions. It is now believed that chondrin is not a simple body, but a mixture of 

 gelatin with mucinoid substances, chief among which, perhaps, is a compound 

 termed chondro-mucoid. 



FIG. 623. Yellow cartilage, ear of horse. High power. 



