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BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



com, dg. 



ext. prp. ex. 

 ext. prp. i. 



has shifted towanl the iihiar side. The supernumerary metacarpal (ii) 

 is the smallest of the three ; it is fused to iii througliout its whole 

 length, and can be traced to the proximal extremity of the metacarpus, 

 where it takes part in forming the articular facet for the carpals. The 



distal epiphysis extends beyond those of 

 the normal metacarpals, has a flattened 

 instead of a convex articular surface, and 

 no trochlear ridge. The fifth metacarpal 

 (Fig. IF, v) is, as normally, a rudimentary 

 stylet articulating at the ulnar side of the 

 proximal extremity of iv. 



All three digits are composed of three 

 phalanges. Digit iv is apparently normal ; 

 digit III is more massive, and the sym- 

 metry of its phalanges and hoof is affected 

 by the presence of the abnormal digit. 

 Instead of being optical images of those 

 of digit IV, these bones are indifferent in 

 their conformation, curving neither to the 

 right nor to the left. The hoof in Avhich 

 the ungual phalanx is enclosed is coniraou 

 also to digit ii. The extra digit (ii) is 

 shorter and not so massive as the normal 

 ones ; its ungual phalanx is flattened lat- 

 erally, and more pointed than the normal 

 phalanges ; the sesamoitls are absent. 



Dissection of the musculature of this 

 manus shows that the flexors are entirely 

 normal ; the extensors, however, exhibit an 

 important modification. The tendon of the 

 extensor proprius internus (Fig. W, ext. 

 prp.i.) divides, and the more radial of the 

 two slips thus formed is inserted into the 

 second and-ungual phalanges of the super- 

 numerary digit. Before its insertion this 

 tendon is joined by a division of the suspensory ligament. The anatom- 

 ical relations of this tendon thus resemble the normal condition in 

 four-toed animals. If the supernumerary digit is a duplication of digit 

 III, we sliould expect to find the extensor communis digitorum (ext. 

 coin, dg.) and the flexor tendons bifurcated ; but they are unmodified. 



II. III. IV. 



Fig. W. — Anterior view of the 

 left polydactylc niamis of a calf, 

 sliowing the extensor muscles, ii, 

 supernumerary digit ; v, nietacar- 

 pai of digit five; ext. com. dg., 

 extensor communis digitorum ; ext. 

 p7-p. ex., extensor proprius exter- 

 nus; ext. prp. i., extensor pro- 

 prius internus. i natural size. 



