106 CONTRACTILE ELEMENTS. 



have little motion. But in the movable articulations (diar- 

 throses), the ligaments, situated principally in the periphery, 

 are powerless to prevent the disturbance of the articulating 

 surface, as may easily be seen in the scapulo-humeral and 

 coxo-femoral articulations, where the heads of the bones 

 may be considerably displaced from the socket, in spite of 

 the perfection of the ligamentous system. In joints of this 

 kind it is simply atmospheric pressure (Weber) which pro- 

 duces the adhesion of the articulating surface. A dead body 

 may be suspended, the lower limbs hanging freely, and we 

 may then remove all the soft parts, skin and muscle, which 

 surround the coxo-femoral joint; the articulating capsule 

 may then be cut, and the limb will still remain suspended 

 from the cotyloid cavity : an additional weight may even be 

 superimposed without destroying the adhesion ; but if, by an 

 opening made in the bottom of the cotyloid cavity, the air 

 is allowed to penetrate the articulating surface, the adhesion 

 ceases instantaneously, and the head of the femoral bone 

 quits its socket. If the bones be then replaced and the air 

 which has entered expelled, and if the opening previously 

 made be stopped up with the finger, the limb will again 

 remain suspended as long as the air is kept out (experiment 

 of the brothers Weber). It is thus the vacuum, or the close 

 contact of surfaces, which allows the atmospheric pressure to 

 act as a counterpoise to the limbs, that are thus supported 

 without any aid from the muscles. 



When, by stretching the fingers, we succeed in slightly 

 separating the phalanges, a well-known crackling sound is 

 produced, of which the foregoing study supplies an explana- 

 tion: the stretching of the joints of the phalanges overcomes 

 the pressure of the atmosphere, and separates the articulat- 

 ing surfaces which were kept in contact by it ; but, at the 

 moment of separation, the soft peripheric parts are thrown 

 by the same pressure into the space between the two bones. 

 These phenomena are very sudden, and give rise to sonorous 

 vibrations, whence the crackling sound. 



The preceding remarks on the mechanism of the bones, 

 of the muscles, and of the tendons, help us to understand 

 the different kinds of labor and the different movements of 

 which man is capable. We need not examine the action of 

 jumping, of climbing, of swimming, etc. We will only con- 

 sider, for a moment, the ordinary walking step, the brothers 

 Weber having shown that in this mode of progression each 



