238 ANIMAL MECHANICS. 



As the limb swings, the horizontal straight line on the board will be 

 found to oscillate about a definite point in the line of the pencil, provided 

 this line runs through the centre, and this point will therefore mark the 

 position of the centre. If the horizontal line lies above or below the 

 centre, it will, while the arm swings, alternately intersect the line of the 

 pencil in two points. A line drawn vertically from a point bisecting the 

 line joining these two points will pass through the centre, and this line 

 may be drawn upon the skin. The observer should now hold his pencil 

 parallel with a vertical line, and determine in the same way the position 

 of the centre of the joint in an axis which is horizontal. The point of 

 intersection of the two lines will mark the position of the centre of the 

 joint. It is advisable in such an experiment to support the pencil, not 

 by the hand, but by some kind of stand. In the case of a joint like 

 the shoulder, the shoulder girdle should be steadied by an assistant, and 

 the observer himself should move the arm backwards and forwards. 



Special ball-and-socket joints. — In the shoulder-joint considerable 

 movement can occur. In its ordinary position the humerus can rotate 

 through an angle of 90° round its longitudinal axis. Round a sagittal 

 axis the humerus can rotate through 120°, round a frontal axis through 

 an angle of 170°. By an added movement of the whole shoulder the 

 humerus may be moved through an angle of 170° round a sagittal axis, 

 and through an angle of 190° round a frontal axis. With the com- 

 bined movement of pronation and supination of the bones of the lower 

 arm, the hand in all can be rotated through an angle of 280°. 



In the case of the hip-joint we have a smaller range of movement, 

 due in the first place to the capsular structure of the joint. This power- 

 ful and very inextensible structure consists of a capsule, strengthened I >y 

 accessory ligaments, formed many of them by fascial sheaths of the 

 surrounding muscles which are attached to it. The most important of 

 these is the ileo-femoral ligament, which hinders eversion and too great 

 extension of the limb. The actual movements of the limb at this joint 

 are likewise hampered by the thick muscles of the hip, either by their 

 passive resistance to pressure or by their extensibility. The femur 

 can rotate through an angle of 45° round an axis running in the length 

 of the limb. Around a sagittal axis rotation can take place through 

 an angle of 80°, and round a frontal axis it can rotate through an angle 

 of 140°. 



The lower limb is not so movable upon the trunk as is the upper 

 one, not only on account of the more limited movement of the hip, 

 as compared with the shoulder-joint, but also because the acetabular 

 surface occupies a fixed position on the trunk, whereas the glenoid cavity 

 is itself movable. 



Hinge-joints. — In these cases one articular surface forms a portion 

 of a cylinder, and we may conceive it as having been generated by the 

 rotation of a straight line around a central axis. This articular cylinder 

 is received into another concave surface of similar shape. The surfaces 

 are not always true cylinders, in fact more often grooved, or of a conical 

 shape. In hinge-joints the capsular ligaments are characterised by 

 greater strength at the sides, and are here termed the lateral ligaments 

 of the joint. Rotation can occur around a single axis, every portion of 

 the surface of the joint describing in its movements a portion of a 

 circle. In these joints the axis is rarely at right angles with the long 

 axis of the bones concerned; thus the trochlear surface and axis 



