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a THE DIFFERENT KINDS OF MOVEMENT AT JOINTS. 
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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 about 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 be 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.t 
At the ankle-joint the natural attitude of the foot is flexion at a right angle to the leg. The 
diminution of this angle by approximating the dorsum of the foot towards the front of the leg 
constitutes flexion ; while any effort at placing the foot and leg ina straight line, ze. 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 mesial 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 the foregoing, and signifies movement towards the 
mesial 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. 
THE DEVELOPMENT OF JOINTS. 
Just as the question of structure determines to a large extent the presence or absence 
of movement in joints, so in tracing their development it will be found that the 
1 From this it will be seen that the term “‘over-extension” is not a correct expression, and conse- 
quently should not be employed. 
