222 THE ARTICULATIONS 



G-liding movement is the most simple kind of motion that can take place in a 

 joint, one surface gliding or moving over another without any angular or rotatory 

 movement. It is common to all movable joints, but in some, as in the articu- 

 lations of the carpus and tarsus, it is the only motion permitted. This movement 

 is not confined to plane surfaces, but may exist between any two contiguous 

 surfaces, of Avhatever form, limited by the ligaments which enclose the articu- 

 lation. 



Angular movement occurs only between the long bones, and by it the angle 

 between the two bones is increased or diminished. It may take place in four 

 directions : forward and backward, constituting flexion and extension, or inward 

 and outward, from the mesial line of the body (or in the fingers and toes from 

 the middle line of the hand or foot), constituting adduction and abduction. The 

 strictly ginglymoid or hinge-joints admit of flexion and extension only. Abduction 

 and adduction, combined Avith flexion and extension, are met Avith in the more 

 movable joints; as in the hip, shoulder, and metacarpal joint of the thumb, and 

 partially in the wrist. 



Circumduction is that limited degree of motion which takes place betAveen the 

 head of a bone and its articular cavity, Avhilst the extremity and sides of the limb 

 are made to circumscribe a conical space, the base of which corresponds Avith the 

 inferior extremity of the limb, the apex with the articular cavity ; this kind of 

 motion is best seen in the shoulder- and hip-joints. 



Rotation is the movement of a bone upon an axis, which is the axis of the pivot 

 on which the bone turns, as in the articulation between the atlas and axis, Avhen 

 the odontoid process serves as a pivot around which the atlas turns ; or else is the 

 axis of a pivot-like process Avhich turns within a ring, as in the rotation of the 

 radius upon the humerus. 



Ligamentous Action of Muscles. The movements of the different joints of a 

 limb are combined by means of the long muscles Avhich pass over more than one 

 joint, and which, Avhen relaxed and stretched to their greatest extent, act as 

 elastic ligaments in restraining certain movements of one joint, except when 

 combined with corresponding movements of the other, these latter movements 

 being usually in the opposite direction. Thus the shortness of the hamstring 

 muscles prevents complete flexion of the hip, unless the knee-joint is also flexed, 

 so as to bring their attachments nearer together. The uses of this arrangement 

 are threefold : 1. It co-ordinates the kinds of movement which are the most 

 habitual and necessary, and enables them to be performed with the least expendi- 

 ture of power. " Thus in the usual gesture of the arms, whether in grasping or 

 rejecting, the shoulder and the elbow are flexed simultaneously, and simultaneously 

 extended," in consequence of the passage of the Biceps and Triceps cubiti over both 

 joints. 2. It enables the short muscles which pass over only one joint to act upon 

 more than one. " Thus, if the Rectus femoris remain tonically of such length 

 that, Avhen stretched over the extended hip, it compels extension of the knee, then 

 the Gluteus maximus becomes not only an extensor of the hip, but an extensor 

 of the knee as well." 3. It provides the joints with ligaments which, Avhile they 

 are of very great power in resisting movements to an extent incompatible Avith the 

 mechanism of the joint, at the same time spontaneously yield when necessary. 

 " Taxed beyond its strength, a ligament Avill be ruptured, whereas a contracted 

 muscle is easily relaxed; also, if neighboring joints be united by ligaments, the 

 amount of flexion or extension of each must remain in constant proportion to that 

 of the other ; Avhile, if the union be by muscles, the separation of the points of attach- 

 ment of those muscles may vary considerably in different varieties of movement, 

 the muscles adapting themselves tonically to the length required." The quotations 

 are from a very interesting paper by Dr. Cleland in the Journal of Anatomy and 

 Physiology, No. 1, 1866, p. 85; by whom I believe this important fact in the 

 mechanism of joints Avas first clearly pointed out, though it has been independently 

 observed afterward by other anatomists. Dr. W. W. Keen points out how 

 important it is " that the surgeon should remember this ligamentous action of 



