PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 277 



A still greater development of digit i was exhibited in two of the 

 cases studied. Such a mauus is shown in Figure 17 (Plate 9). The three 

 phalanges and metacarpal of digit i are larger thau those of digit ii ; 

 the digit is borne on the trapezium, which is also large and articulates 

 "with the scaphoid and trapezoid. The other skeletal elements of the 

 manus are normal in structui'e. The musculature and innervation 

 of these two cases were identical with those shown in Figures 0, P, 

 and L. 



The cases thus far described possess but one extra digit. Continuing 

 the examination of the polydactyle series, it is found that this digit may 

 be partially or completely doubled. 



b. Two Supernumerary Digits. 



Ten cases were studied. Fi'om the intermediate conditions found, it 

 seems probable that tliese forms of polydactylism are further modifica- 

 tions of those instances which have but a single extra digit. Figure 18 

 (Plate 10) shows the skeletal structure of one of the simplest of these 

 conditions. Tlie anatomy of the manus resembles in general that seen in 

 Figure 17 (Plate 9). Metacarpal i is large and articulates with the tra- 

 pezium, but instead of a single set of phalanges two series of bones are 

 present. One of these series (Plate 11, Fig. 19, i*) may be small, pollex- 

 like and composed of two phalanges, or both sets may be of nearly equal 

 size and each consist of three elements (Plate 10, Fig. 18, i", i''). Of 

 four cases examined, three showed the latter condition. The trapezium 

 and scaphoid are abnormally large in all cases. The musculature is like 

 that of the pentadactyle manus (Figs. 0, P), but the tendons which there 

 supply the single extra digit may here bifurcate, and be inserted into the 

 two digits. The nerve branch which supplies the first digit in Figure L 

 also divides (Fig. Q), so that in these cases there is undoubtedly a dupli- 

 cation of digit I. Eliminating this digit, the x'est of the manus, save for 

 the large size of the trapezium, would be entirely normal. 



We now pass to a polydactyle condition in which digit i is completely 

 divided. The manus shown in Figure 20 (Plate 12) is interesting as 

 being a stage intermediate between the preceding cases and a complete 

 hexadactyle condition, and as additional evidence that the two exti-a 

 digits are produced by the duplication of digit i. For in this case, al- 

 though each is composed of a metacarpal and three phalanges, i" and 

 i** are alike in size and form ; still more noteworthy is the fact that the 

 two ungual phalanges are enveloped in a single hoof, and that the two 

 metacarpals articulate with the single trapezium. This carpal is large ; 



