330 HUMAN ANATOMY. 



it to the head of the metacarpal bone of the index-finger is shortened ; (5) the 

 antero-posterior diameter of the wrist is increased; (6) the flexion and immobihty 

 of the wrist are greater. 



In dislocation forzvard the posterior swelling (the sharp border of the radius 

 and ulna) approaches the hand ; the rounded prominence of the carpus is on the 

 front of the wrist ; the antero-posterior diameter is increased and the stylo-meta- 

 carpal measurement is lessened. 



Outward (radial) dislocation of the wrist is resisted by the contact of the 

 scaphoid with the styloid process of the radius and by the internal lateral ligament. 

 Inward dislocation would theoretically be easier, as there is no bony obstacle, and 

 as adduction may be effected to a greater extent than abduction, and with greater 

 power, on account of the leverage afforded by the projection of the cuneiform and 

 pisiform bones on the inner side of the wrist. It is for this reason that the hand 

 commonly assumes the position of adduction and the little finger becomes inclined 

 towards the ulna when, from disease or other cause, the muscles lose the influence 

 of volition and exercise an uncontrolled sway over the part (Humphry), Disloca- 

 tion in either lateral direction is, however, very rare. 



Spontaneous subluxation forward is a condition thought to be associated with 

 hard manual labor in which the strong anterior ligament becomes stretched and the 

 radial side of the carpus is displaced forward and upward. This is followed, in 

 accordance with a general law of growth (page 104), by an overgrowth of the 

 posterior portion of the lower end of the radius, from which the normal opposing 

 pressure of the carpus has been removed. The lower end of the ulna becomes 

 unduly prominent. 



Disease of the wrist-joint is frequently tuberculous, but may be septic or rheu- 

 matic or gonorrhoeal in its origin. As the joint-cavity does not include the epiphyseal 

 lines of either the radius or ulna, the synovial membrane being attached to the 

 margins of the epiphyses, disease and injury of the latter do not of necessity involve the 

 joint. The circumstances already detailed that protect the joint from dislocation 

 also protect it from sprains and lessen the frequency of traumatic synovitis and of 

 the sequelae of traumatism. 



Disease of any variety once established is apt to extend to the various synovial 

 pouches of the carpus on account of their proximity, to involve the flexor and ex- 

 tensor tendon sheaths for the same reason, and to result, in accordance with its 

 character, in either extensive disorganization or much limitation of motion. The 

 flexors and extensors on the front and back of the wrist act with about equal force, 

 and therefore but little displacement occurs. 



The swelling usually shows itself first on the dorsum through the thinner pos- 

 terior ligament, the joint being nearer the surface on that aspect. 



Landmarks. — The line of the wrist-joint is convex upward. A straight line 

 drawn between the two styloid processes is oblique downward and outward. It 

 unites the two extremities of the arc which represents the line of the joint. The 

 highest point of that arc is a half-inch above the interstyloid line. 



If a knife were introduced horizontally below the tip of the styloid process of 

 the ulna, it would open the wrist-joint ; below the styloid of the radius, it would 

 Strike the scaphoid. 



The remaining landmarks are described on page 621. 



The Joints of the Carpus, Metacarpus, and Phalanges. — As the inter- 

 mediate ligaments uniting the separate bones of each row of the carpus are all trans- 

 verse, and do not pass from one row to another, the mid-carpal ( intracarpal) joint 

 permits of considerable motion in both flexion and extension. It undergoes disloca- 

 tion with extreme rarity, and usually only as a result of a degree of force sufficient 

 to stretch or tear tendons and ligaments. 



Dislocation of the second row of the carpus forward is prevented by the 

 manner in which the concave surfaces of the trapezium and trapezoid rest upon the 

 posterior convex facet of the scaphoid, as well as by the undulating manner in 

 which the side of the unciform is disposed with regard to the side of the cuneiform. 

 Displacement backward is prevented by the manner in which the round head of the 

 OS magnum and the conve.x posterior and upper surface of the unciform are let into 



