60 TEXT-BOOK OF PHYSIOLOGY. 



left-handed screws. In the knee-joint the form and arrangement 

 of the articulating surfaces are such as to produce that modifica- 

 tion of a simple hinge known as a spiral hinge, or helicoid. As 

 the articulating surfaces of the condyles of the femur increase in 

 convexity from before backward, and as the inner condyle is 

 longer than the outer, and, therefore, represents a spiral surface, 

 the line of translation or the movement of the leg is also a spiral 

 movement. During flexion of the leg there is a simultaneous 

 inward rotation around a vertical axis passing through the outer 

 condyle of the femur; during extension a reverse movement takes 

 place. Moreover, the slightly concave articulating surfaces of 

 the tibia do not revolve around a single fixed transverse axis, as 

 in the elbow- joint, for during flexion they slide backward, during 

 extension forward, around a shifting axis, which varies in posi- 

 tion with the point of contact. 



In some few instances the axis of rotation of the articulating 

 surface is parallel with rather than transverse to the long axis of 

 the bone, and as the movement then takes place around a more 

 or less conic surface, the joint is termed a trochoid or pulley e. g., 

 the odonto-atlantal and the radio-ulnar. In the former the collar 

 formed by the atlas and its transverse ligament rotates around 

 the vertical odontoid process of the axis. In the latter the head 

 of the radius revolves around its own long axis upon the ulna, 

 giving rise to the movements of pronation and supination of the 

 hand. The axis around which these two movements take place 

 is continued through the head of the radius to the styloid process 

 of the ulna. 



2. Biaxial Joints. In this group the articulating surfaces are un- 



equally curved, though intersecting each other. When the sur- 

 faces lie in the same direction, the joint is termed an ovoid joint 

 e. g., the radio-carpal and the atlanto-occipital. As the axes 

 of these surfaces are vertical to each other, the movements per- 

 mitted by the former joint are flexion, extension, adduction, and 

 abduction, combined with a slight amount of circumduction ; 

 the latter joint permits of flexion and extension of the head, with 

 inclination to either side. When the surfaces do not take the 

 same direction, the joint, from its resemblance to the surfaces of 

 a saddle, is termed a saddle- joint e. g., the trapezio-metacarpal. 

 The movements permitted by this joint are also flexion, exten- 

 sion, adduction, abduction, and circumduction. 



3. Polyaxial Joints. In this group the convex articulating surface 



is a segment of a sphere, which is received by a socket formed 

 by the opposing articulating surface. In such a joint, termed an 

 enarthrodial or ball-and-socket joint, e. g., the shoulder- joint, 

 hip-joint, the distal bone revolves around an indefinite number 



