136 LOCOMOTION. [CHAP. vi. 



a cone whose apex is the articular extremity moving in the joint ; 

 this motion is called circumduction. 



Rotation is simply the revolving of a bone around its axis. It is 

 important to bear this definition in mind : through losing sight of it, 

 many anatomists have attributed rotation to a joint which really 

 does not possess it. 



The varieties of the diarthrodial joint are as follows : 

 a. Arthrodia. In this species the surfaces are plane, or one is 

 slightly concave, and the other slightly convex : the motion is that 

 of gliding, limited in extent and direction only by the ligaments of 

 the joint, or by some process or processes connected with the bones. 

 The examples in man are, the articular processes of the vertebrae, 

 the radio- carpal, carpal, carpo-metacarpal, inferior radio-ulnar, supe- 

 rior tibio-fibular, tarsal and tarso-metatarsal, temporo-maxillary, 

 acromio-clavicular, and ster no -clavicular joints. This last articula- 

 tion and the wrist-joint possess a greater latitude of motion than 

 the others ; the former, in consequence of the shape of its articular 

 surfaces ; each surface is convex in one diameter and concave in the 

 other, so that the gliding that takes place in this joint is in the 

 direction of the long and short diameters, which intersect each other 

 at right angles. It is capable, therefore, of vague opposition in 

 those lines, but certainly not in the intermediate directions, the 

 nature of the surfaces being calculated to prevent this. The wrist 

 owes its mobility to the laxity of its ligaments, which permit it to 

 move as well in its transverse as in its antero-posterior diameters, 

 as also in the intermediate directions ; it consequently admits of 

 vague opposition and circumduction. The articulation of the meta- 

 carpal bone of the thumb with the trapezium is also an arthrodia 

 very similar to the sterno-clavicular, but with a greater degree of 

 motion. Arthrodial joints are generally provided with ligaments, 

 placed at the extremities of the lines in the direction of which the 

 gliding motion takes place. 



b. Enarthrosis. This is a highly developed arthrodia. The 

 convex surface assumes a globular shape, and the concavity is so 

 much deepened as to be cup-like ; hence the appellation ball and 

 socket. The ball is kept in apposition wtih the socket by means of 

 a capsular ligament, which is sometimes strengthened by accessory 

 fibres at certain parts that are likely to be much pressed upon. 

 The best example of enarthrosis is the hip-joint, and next to it the 

 shoulder : in the latter the cavity is but imperfectly developed. 

 All the quadrupeds have their shoulder and hip-joints on this con- 

 struction, and the same common plan is observed in the vertebrata 



