582 MECHANISM OF THE BONES AND THEIR ATTACHMENTS. 



1. Joints with a Rotatory Movement about One Axis. (a) The hinge-joint or 

 ginglymus. The one articular surface represents a section of a cylinder or cone, 

 about one axis of which the other surface, with a corresponding concavity, moves 

 on flexion or extension at the joint. Examples: the joints of the fingers and 

 the toes. Strong lateral supporting ligaments are always present, to prevent 

 lateral flexion of the joint. 



The screw-hinge joint is a modification of the hinge- joint. The humero-ulnar 

 articulation belongs to this class. Strictly speaking, simple flexion and extension 

 do not take place at the elbow-joint; but the ulna is rotated on the trochlea of 

 the humerus like a nut on a bolt; on the right humerus the screw is wound to 

 the right, and on the left humerus to the left. The ankle-joint also belongs to 

 this class; the nut is the articular surface of the tibia; the right joint resembles 

 a left-handed screw, the left joint the reverse. 



(6) The pivot- joint (rotatio) , with a cylindrical articular surface ; for example , 

 the articulation between the atlas and the odontoid process of the axis, which 

 represents the axis of rotation. The axis of rotation of the articulation at the 

 elbow- joint for pronation and supination extends from the middle of the cotyloid 

 cavity on the head of the radius to the styloid process of the ulna. Accessory 

 joints for this pivot- joint are, above, the articulation between the articular cir- 

 cumference of the head of the radius and the lesser sigmoid cavity of the ulna; 

 and, below, the articulation between the head of the ulna and the sigmoid cavity 

 of the radius. 



2. Joints with a Rotatory Movement about Two Axes. (a) The joints exhibit 

 in the two axes, which intersect at right angles, a curvature that is different in 

 degree, but the same in direction: for example, the atlanto-occipital articulation, 

 or the wrist-joint, in which both flexion and extension, as well as lateral inclination, 

 are possible. (6) The joints have a surface of curvature that pursues a different 

 direction in the two axes, which intersect at right angles. To this class belongs 

 the saddle- joint, the surface of which is concave in the direction of the one axis, 

 and convex in that of the other; for example, the' articulation between the tra- 

 pezium and the metacarpal bone of the thumb. The principal movements, here 

 are (i) flexion and extension, and (2) abduction and adduction. Further, move- 

 ment is possible to a limited degree in all other directions, and a cone-shaped 

 space can be circumscribed by the thumb. In this manner the saddle- joint re- 

 sembles a limited arthrodial joint. 



3. Joints with a Movement on a Spiral Articular Surface (Spiral Joints}. To 

 this class belongs above all the knee-joint. The condyles of the femur, curved 

 from before backward, exhibit, on sagittal section of their articular surface, a 

 spiral the center of which lies toward the posterior portion of the condyle, and 

 whose radius vector increases from behind downward and forward. The joint 

 permits, first of all, extension and flexion. The strong lateral ligaments on both 

 sides arise from the condyles of the femur, at a point corresponding to the center 

 of the spiral, and are inserted on the head of the fibula and the internal condyle 

 of the tibia respectively. When the knee-joint is strongly flexed, the lateral liga- 

 ments are relaxed; they become tense as extension increases, and in complete 

 extension they form tense bands, which ensure lateral fixation of the knee-joint. 

 In accordance with the spiral form of the articular surface, flexion and extension 

 do not occur about one axis, but the axis constantly shifts with the points of 

 contact; the axis traverses a path that likewise is sp'iral. The greatest flexion 

 and extension cover about 145. The anterior crucial ligament is made more 

 tense during extension, and acts as a check-ligament for excessive extension; the 

 posterior crucial ligament is made more tense during flexion, and is a check- 

 ligament for excessive flexion. The movements of extension and flexion at the 

 knee are, however, rendered more complex by the screw-like movement of the 

 joint, with the result that the leg deviates outward during extreme extension. 

 Accordingly, the thigh must be rotated outward during flexion, if the leg is fixed. 

 Pronation and supination further are observed in the knee-joint, amounting to 

 41 in extreme flexion, but being entirely absent in extreme extension. They are 

 due to rotation of the external condyle of the tibia about the internal condyle. 

 In all positions of flexion the crucial ligaments exhibit a fairly uniform degree 

 of tension, as a result of which the articular extremities are held in apposition. 

 It is owing to their arrangement that with increase in the tension of the anterior 

 ligament during extension the condyles of the femur must roll more on the anterior 

 portion of the articular surface of the tibia; while with increase in the tension 

 of the posterior ligament during flexion they must roll more on the posterior 



