VARIETIES OF TRUE JOINTS. 113 



nomenclature is generally accepted, since the digital extensor muscles flex and the 

 flexors extend. It is best with reference to the ankle-joint to speak of pla7itar 

 Jiexio7i and dorsal fie x ion. Pro7iation in the arm is turning the front of a limb 

 downward ; supination, the converse. Thus, when the palm rests upon a table the 

 arm is pronated ; when the back of the hand rests upon the same support the arm 

 is supinated. Reference to .the skeleton during these movements will show that pro- 

 nation is associated with crossing of the bones of the forearm, while during supina- 

 tion they are parallel. These terms should not be applied to motions of the leg. 

 Rotation is inward or outward, according as it is towards or away from the median 

 line of the body. 



Varieties of True Joints. — The following are the chief kinds of true joints, 

 the nature of the motion being determined by the articular surfaces : 



Arthrodia,' a gliding joint permitting merely a sliding between two nearly 

 plane surfaces, as betw-een the articular processes of the vertebrae. 



Enarthrosis,'^ a ball-and-socket joiyit permitting ayigular motion in any direc- 

 tion, circumduction and rotation. The shoulder- and hip-joints are conspicuous 

 examples. 



Condylarthrosis,'' an egg-shaped joint permitting angular motions more freely 

 on the long axis than on the short one, circumduction but (theoretically, at least) 

 no rotation, as in the radio-carpal articulations. The imaginary axes for the angular 

 motions lie in the convex bone. 



The Saddle-Joint,* is a modification of the above, the end of one bone being 

 convex in one plane and concave in another, at right angles to the first, while the 

 other bone is the converse ; thus in one plane one bone is the receiver and in the 

 other the received. The articulation of the trapezium with the first metacarpel bone 

 is an example. The motions in such joints are precisely the same as those of 

 the preceding form. The two imaginary axes are, however, on opposite sides of the 

 joint, each being at right angles to the con\'ex plane of its own bone. It is clear 

 that if the reciprocal curves of the two bones of a saddle-joint coincide, and that if 

 they fit closely, rotation is out of the question ; but, in point of fact, that is not the 

 case, for there is no very accurate agreement of the surfaces, and the contained 

 curve is smaller than the containing, so that a certain amount of rotation is possible.* 



Ginglymus,*' a hinge-joint permitting motion only on a single axis approxi- 

 mately transverse to the long axis of the bone, consequently the moving bone keeps 

 in one plane. The ankle-joint is an example. The inclination of the transverse 

 axis may vary, and one end of the joint be larger than the other. If the course of 

 the revolving bone is that of a spiral around the transverse cylinder the articulation 

 constiutes a screzv joint,'' as the humero-ulnar articulation. 



Trochoides,* a pivot-joint permitting motion only on one axis coincident with 

 at least a part of the long axis of the bone, — namely, rotatioji^ as in the atlanto-axial 

 articulation. Should a part of the bone be so bent as to lie outside of the axis, as 

 in the radius, this part undoubtedly changes position ; nevertheless, there is merely 

 rotation, for the change of position is accidental, depending on the shape of the 

 bone, not on the nature of the motion. 



Certain complicated joints may combine several of the above forms. 

 ^Rene du Bois-Reymond. Archiv fiir Anat. u. Phys., Phys. Abtheil., 1895. 



' Arthrodia. - Enarthrosis. ^ Articulatio ellipsoidea. * Articulatio sellaris. * Ciaglymus. " Articulatio cochlearis. 

 ° Articulatio trochoidea. 



s 



