MOVEMENTS OF JOINTS. 131 



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 posi- 

 tion, anterior, posterior, internal, or external lateral. The lateral liga- 

 ments 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 

 ihe required extent of movement. 



Enarthrosis (l\> in, aigdguffis) is the most extensive in its range of motion 

 of all the movable, joints. From the manner of connexion 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-metacar- 

 pal 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 con- 

 cerned, 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 ligamentous 

 membrane, called a capsular ligament. 



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

 factory manner in the tabular form, thus: 



f Sutura - - - - bones of the skull. 



, j Harmonia - - - superior maxillary bones. 



S1S ' 1 Schindylesis - - vomer with rostrum. 



[ Gomphosis - - - teeth with alveoli. 



Amphi-arthrosis - Bodies of the vertebra - Symphyses. 



C Arthrodia - - - carpal and tarsal bones. 



Diarthrosis. < Ginglymus - - - elbow, wrist, knee, ankle. 



( Enarthrosis - - - hip, shoulder. 



The motions permitted in joints may be referred to four heads, viz. 1. 

 Gliding. 2. Angular movement. 3. Circumduction. 4. Rotation. 



1. Gliding is the simple movement of one articular surface upon an- 

 other, and exists to a greater or less extent in all the joints. In the least 

 movable joints, as in the carpus and tarsus, this is the only motion which is 

 permitted. 



2. Angular movement may be performed in four different directions, 

 either forwards and backwards, as in flexion and extension ; or inwards 

 and outwards, constituting adduction and abduction. Flexion and exten- 

 sion are illustrated in the ginglymoid joint, and exist in a large proportion 

 of the joints of the body. Adduction and abduction conjoined with flexion 

 and extension, are met with complete, only in the most movable joints, 

 as in the shoulder, the hip, and the thumb. In the wrist and in the ankle 

 adduction and abduction are only partial. 



3. Circumduction is most strikingly exhibited in the shoulder and hip- 

 joints ; it consists in the slight degree of motion which takes place in the 

 head of a bone against its articular cavity, while the extremity of the limb 

 is made to describe a large circle upon a plane surface. It is also seen, 

 but in a less degree, in the carpo-metacarpal articulation of the thumb, 

 metacarpo-phalangeal articulations of the fingers and toes, and in the 

 elbow when that joint is flexed and the end of the humerus fixed. 



