270 bullf:tin: museum of compauative zoology. 



dactylo animals ; the oblique extensor of the metacarpus might bo found 

 inserted into metacarpal i, and the flexor perforans muscle might send 

 a tendon to digit i. The pollex, if tlius supplied with muscles, should be 

 innervated by a branch from the radial side of the median nerve. In 

 examining tlie following cases of polydactylism in the manus of the pig, 

 we sliall see whetlier these theoretical conditions are ever fulliilcd. 



Of the thirty-six instances of polydactylism which were studied, all 

 were of the manus ; in every case, also, the supernumerary digit oc- 

 curred on the radial side of the extremity. Digit ii is a])n(iniial in some 

 cases. The abnormalities might be divided into numerous types accord- 

 ing to the number and condition of the extra digits ; but as these types 

 grade into one another, we shall attempt to distinguish but two classes : 

 (1) cases in which the supernumerary parts are distinct from, and inde- 

 pendent of, the normal digits; (2) cases, where tliey are more or less 

 closely connected with digit ii. We shall see that even these are artifi- 

 cial groups, and that intermediate conditions link together the two. In 

 the following descriptions, we shall begin with the simplest forms, and 

 pass iu succession to the more complex types of polydactylism. 



1. Manus in which the Super mimerary Digits are Independent of the 



Normal Digits. 



a. One Supernumerary Digit. 



The simplest example of this condition is represented by a single case 

 (Plate 4, Fig. 12). Externally the extra digit (i) is inconspicuous, but 

 originally bore a small claw-like hoof. It is composed of two rudimen- 

 tary phalanges and a spheroidal element, which apparently represents the 

 distal end of a metacarpal. This does not articulate with the second 

 metacarpal, but is merely held in place by fibrous tissue and the skin. 



In the carpus the trapezium is abnormally long ; it articulates with 

 the trapezoid laterally, and has a facet proxiraally for the scaplioid ; in 

 other respects the bones of the manus are normal. The muscles and 

 nerves are unmodified. 



Figure 13 (Plate 5) shows a manus in which the pullox is fully de- 

 veloped. Of this type, four cases were examined. The pollex (i) is 

 smaller than digit ii and consists of the metacarpal and two phalanges. 

 The metacarpal bone articulates with tlie trapezium, which is abnor- 

 mally large and has three facets : a distal for metacarpal i, a latoial 

 for the trapezoid, and a proximal for the scaphoid. The relations of the 

 bones of this digit to those of the rest of the manus are thus identical with 

 the conditions found in fossil swine and in other pentadactyle animals. 



