APPLICATIONS TO PHYSIOLOGY 259 



another, it is the determination of the centres of 

 movement of a joint. 



Determination of the Centres of Movement in 

 Joints. — When two articular surfaces move on one 

 another, the movement does not always take place 

 round a point corresponding to the centre of curvature 

 of the surfaces. We know, for instance, that in the 

 case of the knee-joint the condyles of the femur 

 simultaneously rotate and slide on the articular 

 surfaces of the tibia, and that the condyles of the 

 lower jaw slide in various directions in the glenoid 

 cavity of the temporal bone, etc. It must follow, 

 therefore, that the centre of articular movement is 

 not indicated by the anatomical relations, but must 

 be empirically discovered. 



This problem is almost identical with that which is 

 disposed of in regard to the rolling of ships, and the 

 same means may be employed for the experimental 

 solution. On the cadaver the matter is simple : a 

 hole is drilled in the condyle of the inferior maxilla, 

 and another in the ascending process of the same 

 bone near the lower extremity. A polished metal 

 wire is stretched between these two points ; in the 

 photograph it will be represented as a bright line 

 indicating the axis of the ascending portion of the 

 inferior maxilla. 



The skin of the subject is slightly blackened, and 

 a series of photographs taken on a hxed plate while 

 the jaw is worked up and down. The lines stand out 

 clearly in the photograph, and cross each other at 

 the points which represent the temporary centres of 

 movement of the bone during its angular displacements. 



On the living subject the experiment is not much 

 more difficult, A small " planchette " applied to the 

 teeth of the lower jaw is kept in position by elastic 

 bands passing under the chin. Into this " planchette," 



