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 Ercolani (’81, 
Tav. I, Fig. 2). In this instance digit 1 is of normal size, and its 
metacarpal is fused with metacarpal 1 along its proximal half only. 
This element (1) is large and bears three large phalanges. The com- 
pound bone formed by the fusion of metacarpals 1 and 1 articulates 
above with the trapezoid, which is normal, and also with the trapezium, 
which is abnormally large. If metacarpals 1 and 1 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. O, P, 
pp. 275, 276). In the five cases dissected, digit 11 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, jlx. perf.’), 
which is normally inserted into digit 11, bifurcates and is attached to digit 
1as well. Inall 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. Z, 1, p. 272). There is 
little ground, therefore, for regarding these cases of polydactylism as due 
to duplication of digit 11; on the contrary, there is direct evidence against 
this view. (1) Digit 1 varies in size, while digit m1 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 1, 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. 
b. Two SUPERNUMERARY DIGITS. 
Three cases were observed representing two types. Of the simplest 
condition there was but one case. In this manus digit 1* (Plate 19, 
Fig. 27) consists of two small phalanges and the distal end of a meta- 
