220 THE ARTICULATIONS. 



axes at right angles to each other or at any intervening axis between the two. 

 These are the condyloid and saddle-joint. There is one form of joint where the 

 movement is polyaxial, the enarthrosis or ball-and-socket joint. And finally there 

 are the Arthrodia or Gliding joints. 



Ginglymus or Hinge-joint (yiyytofioi;, a hinge). In this form of joint the 

 articular surfaces are moulded to each other in such a manner as to permit motion 

 only in one plane, forward and backward ; the extent of motion at the same time 

 being considerable. The direction which the distal bone takes in this motion is 

 never in the same plane as that of the axis of the proximal bone, but there is 

 always a certain amount of alteration from the straight line during flexion. The 

 articular surfaces are connected together by strong lateral ligaments, which form 

 their chief bond of union. The most perfect forms of ginglymus are the inter- 

 phalangeal joints and the joint between the humerus and ulna ; the knee and ankle 

 are less perfect, as they allow a slight degree of rotation or lateral movement in 

 certain positions of the limb. 



Trochoides (pivot-joint). Where the movement is limited to rotation, the joint 

 is formed by a pivot-like process turning within a ring, or the ring on the pivot, the 

 ring being formed partly of bone, partly of ligament. In the superior radio-ulnar 

 articulation the ring is formed partly by the lesser sigmoid cavity of the ulna ; in 

 the rest of its extent, by the orbicular ligament ; here the head of the radius 

 rotates Avithin 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 of the atlas ; behind, 

 by the transverse ligament ; here the ring rotates round the odontoid process. 



Condyloid Articulations. In this form of joint an ovoid articular head, or 

 condyle, is received into an elliptical cavity in such a manner as to permit of 

 flexion and extension, adduction and abduction and circumduction, but no axial 

 rotation. The articular surfaces are connected together by anterior, posterior, and 

 lateral ligaments. An example of this form of joint is found in the wrist. 



Articulations by Reciprocal Reception (saddle-joint). In this variety the 

 articular surfaces are concavo-convex ; that is to say, they are inversely convex in 

 one direction and concave in the other. The movements are the same as in the 

 preceding form ; that is to say, there is flexion, extension, adduction, abduction, 

 and circumduction, but no axial rotation. The articular surfaces are connected by 

 a capsular ligament. The best example of this form of joint is the carpo-meta- 

 carpal joint of the thumb. 



Enarthrosis is that form of joint in which the distal bone is capable of motion 

 around an indefinite number of axes which have one common centre. It is formed 

 by the reception of a globular head into a deep cup-like cavity (hence the name 

 " ball-and-socket "), the parts being kept in apposition by a capsular ligament 

 strengthened by accessory ligamentous bands. Examples of this form of articulation 

 are found in the hip and shoulder. 



Arthrodia is that form of joint which admits of a gliding movement; it is 

 formed by the approximation 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 ; as in the articular 

 processes of the vertebrae, the carpal joints, except that of the os magnum with the 

 scaphoid and semilunar bones, and the tarsal joints with the exception of the joint 

 between the astragalus and the navicular. 



On the next page, in a tabular form, are the names, distinctive characters, and 

 examples of the different kinds of articulations. 



The Kinds of Movement admitted in Joints. 



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

 angular movement, 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 we find only one kind of motion in any particular joint. 



