104 CONTRACTILE ELEMENTS. 



plies to the hollow columns formed by the bones of the 

 different members; that is to say, given an equal quan- 

 tity of bony substance, these organs, being filled with little 

 canals (canaliculi), offer greater resistance than if they were 

 solid, and thus combine strength and lightness. 



The bones are not merely rigid levers, necessary to motion ; 

 we have now seen that they serve as columns or supports in 

 standing, intended to sustain the weight of the body. They 

 form also a more or less complete scaffolding for the protec- 

 tion of certain cavities, such as the ribs, the pelvis, and espe- 

 cially the skull, which supplies an incompressible covering to 

 the brain. 



The Joints. The parts by which the different pieces of 

 the skeleton are united are called the joints. The joints are 

 thus, generally, the centres of motion ; they are also placed 

 in such a manner as to avoid friction as much as possible. 

 The cartilages, which cover the surface of the joints, are 

 compressible and elastic, and thus form protecting cushions 

 which serve to moderate shocks, to diminish friction, and to 

 resist pressure in the different movements of locomotion, and 

 hi the equilibrium of the erect posture. They are lubricated 

 by a synovial fluid, which is a ropy and unctuous substance, 

 and, at first sight, resembles the white of egg, but which, by 

 its properties, partakes rather of the nature of mucus, prop- 

 erly so called. Indeed, as is the case with all mucus, the 

 synovial fluid is the result of the liquefaction of an epithe- 

 lium which lines the inner surface of the articulating capsule. 

 The movements and friction between the articulating sur- 

 faces have great influence on the composition of the synovia; 

 when the animal is in repose this fluid is very watery, less 

 sticky than at other times, and contains little cellular waste. 

 After long and active exercise it becomes thick and sticky, 

 containing a larger quantity of synovia, or mucine (see 

 physiology of the mucous surfaces: epitheliums), and of 

 epithelial waste (Frerichs). The synovial fluid thus pos- 

 sesses great power of cohesion, and adheres firmly to any 

 surface which it bathes. Strictly speaking, it is not the car- 

 tilages, but these liquid coats, which come in contact with 

 each other, so that scarcely any friction ensues. It is only 

 in some cases of disease that the synovial fluid disappears, 

 and consequent attrition occurring, causes rapid atrophy 

 and destruction of the adjacent layers of cartilage and 

 bone. 



Around the joints, beside the articulating capsule and its 



