THE MECHANISM OF JOINTS. 233 



The above being the relation of certain given parts of the body to 

 the co-ordinate planes in the position or pose selected for study, any 

 movement of the body will displace these points, which will again take 

 up new positions in relation to these planes. Thus, on bending forwards, 

 the right hip will move out of the frontal plane, and move in the xx 

 direction, say to + 12, and will occupy a lower position in the vertical 

 direction, say 80 cms. from the ground. 



"When the whole body is translated forwards, as in walking or 

 running, these planes must also be conceived of as being translated, one 

 point of. the body, at any rate, preserving its position in one of the 

 co-ordinate planes. 



If it be desired merely to describe a movement of a limb, the method 

 of polar co-ordinates described at the head of p. 230 is more generally 

 employed. If the movement take place in a plane, the position of this 

 plane is noted with reference to three co-ordinate planes, one sagittal, 

 the other frontal, and the third horizontal, all intersecting at the 

 centre of rotation. The plane in which displacement occurs may also 

 be defined by giving the direction of the axis of rotation (which is 

 perpendicular to that plane) with regard to the three co-ordinate planes. 

 Movements not in a plane are either described by resolving them into 

 several movements in different planes, or by compounding the different 

 axes of rotation in the way described on p. 236. 



In addition to the movements hitherto spoken of, in which there is 

 -displacement of a limb, a different kind of movement can take place in 

 some joints, e.g. the hip and shoulder-joint. The limb may remain 

 practically stationary in space while revolving to a certain extent upon 

 its own longitudinal axis. In this case the axis of rotation coincides 

 with the longitudinal axis of the limb. There is no angular displace- 

 ment of the limb itself, but the amount of longitudinal rotation can be 

 measured by the angular displacement of an indicator-rod fixed to the 

 limb perpendicularly to its axis. In the case of the lower limb, the tip 

 of the toes will serve as such an indicator, pointing forward in the 

 ordinary position, and nearly lateral in extreme outward longitudinal 

 rotation of the leg. 



Any actual movement not confined to a plane, as for instance 

 waving the arm in a circle, or any movement in which rotation upon 

 the longitudinal axis is combined with the angular displacement, cannot 

 be accurately described by any definite number of rotations round 

 fixed axes. Such movements are said to take place on shifting or 

 instantaneous axes. 



THE MECHANISM OF JOINTS. 



From a mechanical standpoint we may, with A. Tick, divide joints 

 into two classes. In the first class we may place joints, the bones of 

 which have one fixed position of equilibrium, to which they return as 

 soon as any displacing force is withdrawn. To this class belong the 

 synchondroses, such as the vertebrae, united by white fibro-cartilage, and 

 the amphiarthroses, such as the joints between the tarsal and carpal bones. 

 The attachments of the bones to each other is here so firm that but little 

 movement is possible, and that only by a displacement or deformation of 

 the tissue of the joint, which thus surfers a strain, against which it reacts 

 when released, and resumes its original condition of equilibrium. 



