266 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



only a single facet on its distal end and articulates with metacarpal ii. 

 The pollex is present and is represented in the figure by metacarpal i. 

 Digits II and v are relatively large, especially at their proximal extrem- 

 ities ; II is better developed than v, and occupies the whole distal sur- 

 face of the trapezoid. It also articulates by a small facet with the 

 OS magnum. 



The third metacarpal is longer than any of the others and proximally 

 there is no radial process for articulation with the trapezoid. In general 

 we may say that the digits of the fossil swine are confined chiefly to 

 their own carpal bones, while in the pig of the present day the third' 

 metacarpal has developed a radial process which articulates with the 

 trapezoid and has partially crowded out digit ii. In the same way 

 metacarpal iv has encroached upon the distal articular surface of the 

 unciform, and pushed the fiftli digit to oue side ; the third and fourth 

 digits thus come to occupy most of the carpo-metacarpal articulation in 

 the modern pig, a condition of evident advantage, as it strengthens the 

 joint between the carpus and the functional digits. 



If complete reversion occurs in the skeletal parts of the pig's manus, 

 we should expect to find (1) an extra digit of two plialanges articulating 

 with the trapezium, and (2) metacarpals ii and in articulating with their 

 proper carpal bones (trapezoid and os magnum respectively) ; (3) meta- 

 carpal III should be longer than iv, and without a radial process, and 

 (4) digits II and v should be relatively larger than in the normal manus. 



The normal musculature of the manus is quite complex. We need 

 mention here only those muscles which in the polydactyle manus pre- 

 sent variations from the normal. Anteriorly we have (1) the radial or 

 great extensor of the metacarpus (Fig. //, ext. mCcarp. mag.). This is 

 a large muscle and is inserted by a strong tendon into the proximal 

 end of metacarpal iii ; (2) the ulnar or oblique extensor of the meta- 

 carpus (Fig. II, ext. mCcarp. ob.), a small muscle, the tendon of which 

 crosses that of the magnum obliquely, and is inserted into the proximal 

 end of metacarpal ii ; (3) the extensor communis digitorum internus 

 (ext. com. dg.i.), a large muscle inserted by means of three tendons. 

 The main tendon bifurcates, the radial portion being inserted in the 

 third phalanx of digit ii ; the remaining portion of the tendon runs 

 some distance and again bifurcates, the two branches becoming attached 

 to the ungual phalanges of the third and fourth digits ; (4) the extensor 

 proprius internus (ext. prp. i.), a much smaller muscle than tlie preceding, 

 is inserted by two tendons, the larger going to the radial side of the tliird 

 digit, the smaller to the ungual phalanx of ii ; (5) extensor proprius 



