The movements permitted in joints maybe divided into four kinds: gliding and angular movements, circumduction, and rotation. Frequently these are more or less combined in the various joints, so as to produce an infinite variety, and it is seldom that only one kind of motion is found in any particular joint. Gliding movement is the simplest kind of motion that can take place in a joint, one surface gliding over another without any angular or rotatory movement. It is common to all movable joints; but in some, as in most of the articulations of the carpals and tarsus, it is the only motion permitted. This movement is not confined to plane surfaces, but may take place between any two contiguous surfaces, of whatever form.
Angular movement implies diminution, or increase, of the angle between adjoining bones. Two types of angular movement must be defined, and they occur around axes which are set at right angles to each other. They are : (1) flexion and its opposite, extension, and (2) abduction and its opposite, adduction. Flexion occurs around an axis which is transverse or obliquely transverse, and usually results in the approximation of two morphologically ventral surfaces. This definition is not entirely satisfactory, for its first part does not hold good for the joints of the thumb; and its second part, in the cases of the shoulder, hip and ankle joints. The thumb lies in a plane set at right angles to the plane of the fingers. As a result its dorsal surface is directed laterally, and flexion and extension at each of its joints occur around an anteroposterior axis. At the shoulder-joint flexion carries the arm forwards and medially, and its morphologically ventral surface is brought no nearer to the ventral aspect of the trunk. At the hip-joint, owing to the changes which occur in the early stages of development, flexion approximates the morphologically dorsal aspect of the thigh to the ventral aspect of the trunk. The condition at the ankle-joint is complicated by the fact that the foot is set at a right angle to the leg. Bending movement at the ankle implies a diminution of the angle and is frequently termed flexion. On the other hand it results in the approximation of two morphologically dorsal surfaces and might, with an equal amount of justification, be termed extension. It will avoid confusion and misunderstanding if the self-explanatory terms dorsi-flexion and plantar flexion are used in connection with the movements at the ankle-joint.*
Abduction and adduction occur around a more or less anteroposterior axis, except in the case of the carpometacarpal joint of the thumb, where, for reasons already stated, these movements occur around a transverse axis. The terms imply movements from and to the median plane of the body; except in the cases of the digits, where the plane of reference is the median plane of the middle digit (in the hand) or the second digit (in the foot).
* Flexion has also been defined as the position assumed by the joints of the fetus ‘in utero.’ Such a definition would imply that ‘dorsi-flexion’ of the ankle-joint is really ‘flexion.’ This view is supported by the physiological observation that in reflexes involving all the joints of the lower limb flexion at the hip- and knee-joints is always associated with dorsi-flexion ‘of the ankle-joint, and extension of the hip- and knee-joints with `plantar flexiou’ of the ankle-joint. The definitions based on morphological and physiological criteria are contradictory to each other and this emphasizes the desirability for the use of the makeshift terms suggested in the text.
Circumduction is that form of motion which takes place between the head of a bone and its articular cavity, when the bone is made to circumscribe a conical space; the base of the cone is described by the distal end of the bone, the apex is in the articular cavity; this kind of motion is best seen in the shoulder and hip-joints.
Rotation is a form of movement in which a bone moves round a longitudinal axis; the axis of rotation may lie in a separate bone, as in the case of the pivot formed by the odontoid process of the axis, around which the atlas turns; or a bone may rotate around its own longitudinal axis, as in the rotation of the humerus at the shoulder-joint; or the axis of rotation may be not quite parallel to the long axis of the bone, as in the movement of the radius on the ulna during pronation and supination of the hand, where it is represented by a line connecting the center of the head of the radius with the center of the head of the ulna.
Several factors take part in determining the variety and range of movements which may occur at any joint, but the most important are the shapes of the opposed articular surfaces, and the tension, or otherwise, of the ligaments concerned. The range of movement at any joint may also depend on the position of neighbouring joints. This restriction only occurs in situations where some muscle or group of muscles acts on all the joints in question. Full flexion at the hip-joint is only possible when the knee is flexed, owing to the tension of the hamstring muscles and, for a corresponding reason; full flexion at the wrist is only possible when the fingers are extended.
Nerve supply.-Movable joints are innervated by the nerves of supply to the muscles which act on them, and it is probable that this arrangement establishes local reflex arcs which tend to ensure stability. The part of the articular capsule which is rendered taut on the contraction of a given muscle or group of muscles is innervated by the nerve or nerves supplying their antagonists. For example, the inferior part of the articular capsule of the hip-joint, which is put on the stretch in abduction, is supplied by the obturator nerve. Tension of this part of the capsule produces a reflex contraction of the adductor muscles which, in most cases, is successful in preventing overstretching or tearing of the ligaments.
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