HINGE-JOINTS. 



239 



of rotation of the ulna is set obliquely to the axis of the humerus, 

 salient outwards at 105, so that the forearm, when extended, forms an 

 angle with the upper arm re-entrant outwards. 1 



Many joints which we may, on superficial observation, regard as 

 hinge -joints are really screw - joints ; and we may now discuss the 

 character of the surface of a screw, and the movements which such a 

 surface can make. In moving the nut upon the surface of a screw, or, 

 more simply still, in moving a small body up the thread of a screw, the 

 movement of any particle can be resolved into rotation, and what is 

 termed translation. The body or any portion of the surface of the nut 

 will rotate around the cylindrical axis of the screw, and at the same time 

 it will move in a direction parallel with the axis of this cylinder. 



A screw is really an inclined plane rolled upon the surface of a 

 cylinder, the hypotenuse forming a spiral or helix (see Fig. 127). The 

 " pitch " of the screw is the vertical distance between two coils of the 

 spiral, and the inclination of the screw 2 



is the angle bac. 



The motion of the trochlea in the 

 sigmoid cavity is that of a screw moving 

 in a nut, the inclination of the screw 

 is 8, and in the 140 through which 

 the elbow can pass the translation 

 amounts to 2 '5 mm. In most cases 

 the right elbow is a right- and the left 

 a left-handed screw. 2 



The screw character of a joint like 

 the elbow may be demonstrated in the 

 following manner. Clanip the humerus 

 in such a position that the axis of 

 rotation of the elbow -joint, whose 

 ligamentous attachments are undis- 

 turbed, is vertical, the forearm being 

 extended. Then stretch a thread hori- 

 zontally between two retort stands, and 

 get the eye in such a position that a 



FIG. 127. This figure illustrates the 

 fact that a screw is an inclined 

 plane wrapped around the surface 

 of a cylinder. 



marked spot on the coronoid 



process of the ulna is on a level with the thread. When the limb 

 is flexed, the marked point moves round the axis of rotation, and is also 

 displaced upwards or downwards over the thread towards the radial 

 side of the limb. 



The elbow-joint, as anatomically defined, contains two physiologically 

 distinct articulations. The bones of the forearm together form a hinge- 

 joint with the humerus, but the radius is also attached to the ulna by a 

 similar joint, which allows it to rotate on its axis, its perfectly rounded 

 head remaining in unchanged relation to the articular surface of the 

 humerus. It may be remarked that in the radio-ulnar articulation the 

 axis of rotation coincides with the longitudinal axis of the bone. In 

 this respect the joint between radius and ulna, as well as that between 

 atlas and dens epistrophei, represent a particular type of joint, rather to 

 be compared to the pintles of a rudder than to the common form of 

 hinge. 



Eadius and ulna being united at the wrist in nearly the same way, 



1 Macalister, "A Text-Book of Human Anatomy," p. 144. 

 - Hultkrantr, " Das Ellenbogengelenk," Jena, 1892, S. 24. 



