102 



THE HUMAN MOTOR 



74. Joints : Degrees of Freedom. The name of joints is given 

 to the various methods of union between bones, that, union being 

 made by the juxtaposition of " jointed surfaces." These sur- 

 faces are flat " or cuivcd " (spherical, cylindrical, or oval), their 

 contact necessarily causes friction when the bones are displaced. 

 They are covered with supple, pliable, cartilages of varying 

 thicknesses (fig. 108). 



Bone. 



In addition, two joint heads are sometimes enveloped in a 



fibrous capsule, which is prolonged 

 inwardly by a membrane mois- 

 tened by an alkaline liquid called 

 " synovia." This synovia gives 

 rise to a cohesion of about 

 550 grammes per square centi- 

 metre ( l ). Finally, ligaments, 

 tendons, and muscles surround the 

 joint and give it a certain fixity, 

 without in any way impeding its 

 movements. Thus every kind of 

 joint has definite connexions vary- 

 ing in number (fig. 109). 



When the joints are absolutely 

 FJG. 108. " fixed," the bones work on 



each other by fine denticulation, 



making " sutures " or " synarthrosis " with rigid connexions. 

 When they have a very limited freedom, with a short and large 

 connexion, they are called " amphiarthoroses " or "symphyses" ; 

 thus the vertebrae are superimposed with insertions of thick and 

 large cartilaginous masses. The joints, which have ample and 

 diversified movements, are " diarthroscs " ; for example, the 

 bones of limbs and fingers which act like levers. 



The study of Man, as a machine, must be concerned particularly 

 with that of the diarthrosic joints or the moveable connexions 

 of bones and their degree of mobility. The freest movement has 

 six degrees of freedom, three translations, and three rotations 

 ( 16) ; but the shape of jointed surfaces and the connexions 

 are such that they enable joints to be divided into five or six 

 classes, according to the degree of freedom. The surfaces are 

 most often cylindrical, oval or spherical, that is to say, with 

 one, two or three axes, and with one, two or three degrees of 

 freedom. The systems with only one degree of freedom are said 

 to have complete connexions. 



(*) Cohesion is a molecular attraction, it has been measured, as regards 

 synovia, by Bordier, according to a method of Gay-Lussac's. 



