STRUCTURE OF JOINTS. 263 



class have been determined by the kind of motion permitted in each; they are 

 four in number: Arthrodia, Enarthrosis, Ginglymus, Diarthrosis rotatorius. 



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, temporo-maxillary, sterno- and acromio-clavicular, 

 inferior radio-ulnar, carpal, carpo-metacarpal, superior tibio-fibular, tarsal, and 

 tarso-metatarsal articulations. 



Enarthrosis is that form of joint which is capable of motion in all directions. 

 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. 



Q-inglymus, Hinge-joint (yiyyxvjuoj, 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 two directions, forwards and backwards, the extent of motion at the same 

 time being considerable. 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 elbow and ankle ; the knee is less perfect, 

 as it allows a slight degree of rotation in certain positions of the limb : there 

 are also the metatarso-phalangeal and phalangeal joints in the lower extremity, 

 and the metacarpo-phalangeal and phalangeal joints in the upper extremity. 



Diarthrosis rotatorius (Lateral Ginglymus). 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 liga- 

 ment. 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. In the 

 superior radio-ulnar articulation, the ring is formed partly by the lesser sig- 

 moid cavity of the ulna ; in the rest of its extent, by the orbicular ligament , 

 here, the head of the radius rotates within the ring. 



Subjoined, in a tabular form, are the names, distinctive characters, and ex- 

 amples of the different kinds of articulations. 



