110 ARTICULATIONS. 



partly connected by interosseous ligaments, or by the intervention of 

 an elastic fibro- cartilage which adheres to the ends of both bones. 

 Examples of this articulation are seen in the union between the bodies 

 of the vertebrae, of the sacrum with the coccyx, of the pieces of the 

 sternum, the sacro-iliac and pubic symphyses (<rlv, Qvtiv to grow to- 

 gether), and according to some, of the necks of the ribs, with the 

 transverse processes. 



DIARTHROSIS ($/ through, ci^eairis} is the movable articulation, 

 which constitutes by far the greater number of the joints of the body. 

 The degree of motion in this class has given rise to a subdivision into 

 three genera, Arthrodia, Ginglymus, and Enarthrosis. 



Arlhrodia is the movable joint in which the extent of motion is 

 slight and limited, as in the articulation of the clavicle, of the ribs, 

 articular processes of the vertebrae, axis with the atlas, radius with 

 the ulna, fibula with the tibia, carpal and metacarpal, tarsal and meta- 

 tarsal bones. 



Ginglymus (yiyyXvftos, a hinge), or hinge-joint, is the movement 

 of bones upon each other in two directions only, viz. forwards and 

 backwards ; but the degree of motion may be very considerable. The 

 instances of this form of joint are numerous ; they comprehend the 

 elbow, wrist, metacarpo-phalangeal and phalangeal joints in the upper 

 extremity ; and the knee, ankle, metatarso-phalangeal and phalangeal 

 joints in the lower extremity. The lower jaw may also be admitted 

 into this category, as partaking more of the character of the hinge- 

 joint than of the less movable arthrodia. 



The form of the ginglymoid joint is somewhat quadrilateral, and 

 each of its four sides is provided with a ligament, which is named 

 from its position, anterior, posterior, internal, or external lateral. The 

 lateral ligaments are thick and strong, and are the chief bond of 

 union between the bones. The anterior and posterior are thin and 

 loose in order to permit the required extent of movement. 



Enarthrosis (v in, ei^uerif) is the most extensive in its range of 

 motion of all the movable joints. From the manner of connection 

 and form of the bones in this articulation, it is called the ball and 

 socket-joint. There are two instances in the body, viz. the hip and 

 the shoulder. 



I have been in the habit of adding to the preceding the carpo-meta- 

 carpal articulation of the thumb, although not strictly a ball-and- 

 socket joint, from the great extent of motion which it enjoys, and 

 from the nature of the ligament connecting the bones. As far as the 

 articular surfaces are concerned, it is rather a double than a single 

 ball-and-socket, and the whole of these considerations remove it from 

 the simple arthrodial and ginglymoid groups. 



The ball and socket-joint has a circular form ; and, in place of the 

 four distinct ligaments of the ginglymus, is enclosed in a bag of liga- 

 mentous membrane, called a capsular ligament. 



The kinds of articulation may probably be conveyed in a more sa- 

 tisfactory manner in the tabular form, thus : 



