THE KINDS OF MO VEMENT ADMITTED IN JOINTS 267 



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

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

 lation. Gliding over a wide range, as is seen in the sliding of the patella over 

 the condyles of the femur, is called coaptatior.. 



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 or bending and extension 

 or straightening, or inward toward and outward from the medial line of the body, 

 constituting adduction and abduction. Abduction of a limb is movement away 

 from the medial line of the body. Adduction of a limb is movement toward the 

 medial line of the body. In the fingers and toes the significance of the terms are 

 different; abduction means movement of the fingers away from the middle finger 

 or of the toe away from the second toe; adduction means movement of fingers 

 toward the middle finger or of the toes toward the second toe. The strictly gingly- 

 moid or hinge-joints admit of flexion and extension only. Abduction and adduc- 

 tion, combined with flexion and extension, are met with in the more movable 

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

 in the wrist. When two anterior surfaces are brought nearer together, as by 

 bending the elbow or wrist, we speak of the movement as anterior or ventral 

 flexion. Ventral flexion of the wrist is also called volar or palmar flexion. If 

 two posterior surfaces are brought nearer together, as by bending the knee or 

 extending the wrist, we speak of the movement as posterior or dorsal flexion. 



At the wrist-joint the bending of the ulnar margin of the hand toward the 

 ulnar side of the forearm is ulnar flexion; the bending of the radial margin of 

 the hand toward the radial side of the forearm is radial flexion. 



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

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

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

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

 axis of a pivot-like process which 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 com- 

 bined by means of the long muscles which pass over more than one joint, and which, when 

 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 coordinates 

 the kinds of movement which are the most habitual and necessary, and enables them to be per- 

 formed with the least expenditure 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 brachii 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, when 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, while 

 they are of very great power in resisting movements to an extent incompatible with the mechan- 

 ism of the joint, at the same time spontaneously yield when necessary. "Taxed beyond its 

 strength, a ligament will 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; while, if the union be by muscles,the separation of 

 the points of attachment of those muscles may vary considerably in different varieties of move- 



