234 ANIMAL MECHANICS. 



In the second class are placed those joints which more especially 

 concern us, such as the hinge and ball-and-socket joints. Here, the 

 bones forming the joint may be placed in a great number of positions, 

 in any one of which they can remain during muscular relaxation. 



Joints belonging to the second class are formed by bones whose 

 extremities are covered by a layer of articular cartilage, and they are 

 enclosed in capsules and accessory ligaments of white fibrous tissue, 

 sufficiently lax to permit of a more or less free play of action. Synovial 

 fluid lubricates the joint and diminishes the friction between the 

 articular ends. The friction between two surfaces is proportional to 

 the pressures of these two surfaces unless that pressure be very great. 

 The friction is independent of the extent of the surfaces which touch 

 one another. It is greatly influenced by the lubricating material, being, 

 in the case of the joints, greatly diminished by the synovial fluid ; and 

 it is independent of the velocity of the movement. In the joints the 

 coefficient of friction l (p) is very small, and while movement between 

 the surfaces is occurring, the coefficient of kinetic friction (A') is still 

 less, so that practically it may always be regarded as nil. 



The area of contact at articular surfaces. It is generally 

 believed that the articular surfaces are in close contact with each 

 other, but it would appear that this is only the case when considerable 

 pressure has been applied. Konig 2 was the first to show that the 

 articular surfaces in a joint like the hip are not in contact in the dead 

 subject ; for in frozen sections, where the positions of parts are unaltered, 

 a layer of frozen synovia is always found between them. Konig further 

 showed that if the head of the femur be firmly wired to the acetabulum, 

 the contact produced takes place over but a small area of the articular 

 surfaces. If the pressure applied by Konig at all represented the 

 pressures to which the joint is normally subject under the influence of 

 muscular contraction and of the body weight, the joint would not 

 conform in its movements to a true ball-and-socket joint, where there is 

 extensive contact over two geometrically adapted surfaces. Its surfaces 

 would, in consequence of this, slip or roll over each other. 



Braune and Fischer 3 doubt whether this pressure applied by Konig 

 was sufficient, and they found much more extensive contact when the 

 articular surfaces were pressed together by the help of screws. The 

 action of the pressure is to mould the elastic cartilaginous surfaces 

 upon each other, and to make them conform to pure geometrical figures. 

 It is to be regretted that these experiments are but of a qualitative 

 nature, the pressures applied not having been measured. 



The forces "which resist displacements of the articular surfaces. 

 The articular surfaces are bound together by the capsular and other 

 ligaments of the joint, but these ligaments are only put upon the stretch 

 when the surfaces are forcibly separated from each other or when the 

 joint is bent. A certain separation of the surfaces can take place, 

 therefore, without the intervention of these ligaments. This separation 

 is, however, hindered by other forces which we have now to study. 

 The muscles passing over a joint, and attached to the bones on either 



1 If a body press upon a plane surface with a pressure P, and a force Fis required to 



F 

 move it along that surface, the coefficient of friction (generally written /m) is -- 



2 Deutsche Ztschr. f. Chir., Leipzig, 1873, S. 256. 



3 "Die Bewegungen des Kniegelenks," Leipzig, 1891. 



