32 THE HUMAN BODY. 



The polish on the articular cartilages facilitates their sliding 

 over each other, and lessens the friction of the bony ex- 

 tremities, while their elasticity diminishes the pressure and 

 deadens the shocks to which the joints are subject. The 

 thickness of these cartilages is proportioned to the motion 

 and the pressure which they are designed to support, and it 

 is greatest at the centre of the convexity of protuberances 

 and on the borders of cavities. The articular cartilages 

 never ossify, differing in this respect from those which, as in 

 the thorax for example, maintain continuity and play the 

 part of flexible bones. These last are ossifying cartilages. 

 The others differing in structure, being without vessels, have 

 been compared to the enamel on the teeth. In fact, they 

 are composed, like that enamel and some other analogous 

 productions, of an almost inorganic substance, and mechani- 

 cal injuries are the only ones which they have to fear. 



Wherever in the organism surfaces move over each other, 

 they are covered with a membrane which secretes a fluid, 

 differing in quality according as there is a sliding or rubbing 

 of the organs. In the interior of the joints the membranes 

 are termed synovial membranes, and secrete a fluid called 

 synovia, because its physical characters resemble those of 

 the white of an egg. The synovia is to the joints what oil 

 is to the wheels of a machine; incessantly poured out upon 

 the surfaces, it lubricates them, and renders the movement, 

 already so easy owing to the polish of the cartilages, still 

 more so; it increases the suppleness and elasticity of these 

 last, which if they were not supplied with this oily fluid 

 would soon be worn out, and motion would be impossible. 

 This sometimes results from certain diseases, and sometimes 

 also in old age. 



We have said that the joints are united by ligaments. 

 This term is applied to the bands or membranes com- 

 posed of fibrous tissue, flexible and inextensible. The liga- 

 ments, which are in the form of bands, are sometimes parallel 

 and sometimes interlaced, and placed either between the 

 articular surfaces or around them. In the latter case their 

 internal face is covered with intimately adherent synovial 

 membrane. The ligaments are attached to the bones at a 



