286 SYNDESMOLOGY 



a ring, or a ring on a pivot, the ring being formed partly of bone, partly of ligament. 

 In the proximal radioulnar articulation, the ring is formed by the radial notch 

 of the ulna and the annular ligament; here, the head of the radius rotates within 

 the ring. In the articulation of the odontoid process of the axis with the atlas 

 the ring is formed in front by the anterior arch, and behind by the transverse 

 ligament of the atlas; here, the ring rotates around the odontoid process. 



Condyloid Articulation (articulatio ellipsoid ea) . In this form of joint, an ovoid 

 articular surface, or condyle, is received into an elliptical cavity in such a manner 

 as to permit of flexion, extension, adduction, abduction, and circumduction, but 

 no axial rotation. The wrist-joint is an example of this form of articulation. 



Articulation by Reciprocal Reception (articulatio sellaris; saddle-joint). In this 

 variety the opposing surfaces are reciprocally concavo-convex. The movements 

 are the same as in the preceding form; that is to say, flexion, extension, adduction, 

 abduction, and circumduction are allowed ; but no axial rotation. The best example 

 of this form is the carpometacarpal joint of the thumb. 



Enarthrosis (ball-and-socket joints) . Enarthrosis is a joint in which the distal 

 bone is capable of motion around an indefinite number of axes, which have one 

 common center. It is formed by the reception of a globular head into a cup-like 

 cavity, hence the name "ball-and-socket." Examples of this form of articulation 

 are found in the hip and shoulder. 



Arthrodia (gliding joints} is a joint which admits of only gliding movement; it 

 is formed by the apposition of plane surfaces, or one slightly concave, the other 

 slightly convex, the amount of motion between them being limited by the ligaments 

 or osseous processes surrounding the articulation. It is the form present in the 

 joints between the articular processes of the vertebra?, the carpal joints, except 

 that of the capitate wdth the navicular and lunate, and the tarsal joints with the 

 exception of that between the talus and the navicular. 



THE KINDS OF MOVEMENT ADMITTED IN JOINTS. 



The movements admissible in joints may be divided into four kinds: gliding 

 and angular movements, circumduction, and rotation. These movements are often, 

 however, 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. Gliding movement is the simplest 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 most of the articulations of the carpus and tarsus, it is the only motion per- 

 mitted. This movement is not confined to plane surfaces, but may exist between 

 any two contiguous surfaces, of whatever form. 



Angular Movement. 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: (1) forward and backward, constituting flexion and extension; or (2) 

 toward and from the median plane of the body, or, in the case of the fingers or 

 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. Abduc- 

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

 movable joints; as in the hip, the shoulder, the wrist, and the carpometacarpal 

 joint of the thumb. 



Circumduction. 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, 



