THE DIFFERENT KINDS OF MOVEMENT AT JOINTS. 303 



(subcutaneous mucous bursae) ; (6) between a tendon and some surface, bony or cartilaginous, over 

 which it plays (subtendinous mucous bursae) ; (c) between a tendon or a group of tendons and the 

 walls of osteo-fascial tunnels, in which they play (vaginae mucosae tendinum or mucous sheaths 

 of tendons). Subtendinous mucous bursae are often placed in the neighbourhood of joints, and in 

 such cases it not infrequently happens that there is a direct continuity between the bursa and the 

 synovial stratum which lines the cavity of the joint through an aperture in the articular capsule. 



THE DIFFERENT KINDS OF MOVEMENT AT JOINTS. 



Reference has already been made to the existence of fixed axes of movement as 

 a basis for the classification of certain forms of diarthrodial joints. Hence it is 

 evident that the movements which are possible at any particular joint depend to a 

 large extent upon the shape of its articular surfaces as well as upon the nature of 

 its various ligaments. Therefore the technical terms descriptive of movements 

 either indicate the directions in which they occur, or else the character of the com- 

 pleted movement. 



In the great majority of articulations between short bones, the amount of move- 

 ment is so restricted, and the displacement of the opposing articular surfaces so 

 slight, that the term gliding sufficiently expresses its character. 



A gliding movement of an extensive kind, for example that of the patella upon the femur, in 

 which the movement largely resembles that of the tyre of a wheel revolving in contact with the 

 ground so that different parts are successively adapted to each other, is called co-aptation. 



Articulations between long bones, on the other hand, are usually associated 

 with a much freer range of movement, with a corresponding variety in its character. 

 Rotation is a movement around an axis which is longitudinal. Sometimes it is the 

 only form of movement which a joint possesses ; at other times it is merely one of a 

 series of movements capable of execution at the same joint. Flexion or bending is 

 a movement in which the formation of an angle between two parts of the body is 

 an essential feature. As it is possible to perform this movement in relation to two 

 axes, viz., a transverse and an antero-posterior axis, it is necessary to introduce 

 qualifying terms. Thus, when two anterior or ventral surfaces are approximated, 

 as at the hip-, elbow-, or wrist-joints, the movement is called ventral, anterior, or 

 palmar flexion ; but if posterior or dorsal surfaces are approximated by the process of 

 bending, then the flexion becomes posterior or dorsi-flexion, as at the knee- or wrist- 

 joints. Further, at the wrist-joint, the formation of an angle between the ulnar 

 border of the hand and the corresponding aspect of the forearm, produces ulnar 

 flexion, and similarly the bending of the hand towards the radial border of the 

 forearm is radial flexion. 



Extension or straightening consists in obliterating the angle which resulted from 

 flexion. In the case of certain joints, therefore, such as the elbow, wrist, and knee, 

 the segments of the limb occupy a straight line as regards each other when 

 extended. 



At the ankle-joint the natural attitude of the foot to the leg is flexion at a right angle. The 

 diminution of this angle by approximating the dorsum of the foot towards the anterior aspect 

 of the leg constitutes flexion ; while any effort at placing the foot and leg in a straight line, i.e. 

 obliteration of the angle, as in pointing the toes towards the ground and raising the heel, 

 constitutes extension. 



Abduction is a term which either expresses movement of an entire limb in a 

 direction away from the median plane of the body, or of a digit, away from the 

 plane of the middle finger in the hand, or the plane of the second toe in the case 

 of the foot. 



Adduction is the reverse of abduction, and signifies movement towards the 

 median plane of the body, or towards the planes indicated for the digits. of the hand 

 and foot. 



Circumduction is a movement peculiarly characteristic of multiaxial or ball- 

 and-socket joints. It consists in combining such angular movements as flexion, 

 extension, abduction, and adduction, so as to continue the one into the other, 

 whereby the joint forms the apex of a cone of movement, and the free end of the 

 limb travels through a circle which describes the base of this cone. 



