PRENTISS : POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 285 



and of its first and second metacarpals. This view is borne out by an- 

 other manus, in which the trapezium is fused to the proximal end of the 

 compound metacarpal, and also by a case figured by Ei-colani ('81, 

 Tav. I, Fig. 2). In this instance digit ii is of normal size, and its 

 metacarpal is fused with metacarpal i along its proximal half only. 

 This element (i) is large and bears three large phalanges. The com- 

 pound bone formed by the fusion of metacarpals i and ii articulates 

 above with the trapezoid, which is normal, and also with the trapezium, 

 which is abnormally large. If metacarpals i and ii of the manus shown 

 in Figure 17 were fused at their proximal ends, we should have a con- 

 dition identical with that figured by Ercolani. 



The evidence of the skeletal parts is in the main confirmed by the 

 arrangement of the muscles and nerves. The condition of the muscles 

 is similar to that of cases where the extra digit is distinct (Figs. 0, P, 

 pp. 275, 276). In the five cases dissected, digit ii retained its own pe- 

 culiar muscles. In one case all the muscles were normal ; and in one in- 

 stance the most radial tendon of the flexor perforans (Fig. P, fix. per/.'), 

 which is normally inserted into digit ii, bifurcates and is attached to digit 

 I as well. In all cases the supernumerary digit was innervated by a special 

 branch given off independently from the radial side of the trunk of the 

 median nerve, as in pentadactyle animals (Fig. L, 1, p. 272). There is 

 little ground, therefore, for regarding these cases of polydactylism as due 

 to duplication of digit ii ; on the contrary, there is direct evidence against 

 this view. (1) Digit i varies in size, while digit ii always remains 

 normal; (2) they are not symmetrical with each other; (3) the 

 divisions of the metacarpal bone are unequal ; (4) the trapezoid is not 

 duplicated nor increased in size ; (5) there is no general duplication of 

 muscle tendons ; (6) the extra digit is innervated by an independent 

 branch of the median nerve. 



In favor of the assumption that the extra digit represents the pollex 

 independently developed and later fused to metacarpal ir, is the fact 

 that the trapezium is of abnormal size, and always articulates with the 

 radial portion of the proximal facet of the compound metacarpal ; also 

 the striking resemblance of the skeletal, muscular, and nervous structures 

 to those of the cases in which the extra digit does arise independently. 



h. Two SUPERNUMERART DiGITS. 



Three cases were observed representing two types. Of the simplest 

 condition there was but one case. In this manus digit i» (Plate 19, 

 Fig. 27) consists of two small phalanges and the distal end of a meta- 



