PRENTISS: POLYDACTYLISxM IN MAN AND DOMESTIC ANLMALS. 281 



second phalanges and forms a sheath for each division of the perforans 

 tendon. The innervation is shown in Figure Q. 



With the increase in size of the extra digits of the polydactyle series, 

 goes a corresponding decrease in the size of digit ii. It is apparently 

 rednced, and partially, sometimes completely, atrophied on account of 

 the abnormal development of the supernumerary parts. In a case fig- 

 ured by Bateson ('94) tiie middle portion of metacarpal ii is gone. In 

 two front feet, from a single animal, I found that the left manus was 

 like that shown in Figure 20, the trapezoid and proximal end of meta- 

 carpal II being reduced ; in the right manus, however, metacarpal ii 

 was completely atrophied, but the three plialanges persisted and were 

 of nearly normal size. The trapezoid remained as a small flattened bone, 

 articulating chiefly with metacarpal in. The reduction is carried a 

 step further in another case, in which the three phalanges of digit ii 

 are present, but exceedingly small, and the hoof reduced to a claw-like 

 vestige (Plate 14, Fig. 22, ii). 



The nerve branch which normally supplies the second digit innervates 

 this vestige (Fig. T, 2), making it reasonably certain that we have to do 

 with the rudiment of digit ii. 



Figure 23 (Plate 15) represents the skeletal parts of a manus in 

 which the second digit has apparently atrophied completely. Three 

 specimens were examined which exhibited this condition. Such cases 

 have been described as dujjlications of digit ii, but a cai'eful study of 

 the manus shows that this is not the case. If we compare Figure 23 

 with Figure 22, the resemblance between the skeletal parts of the extra 

 digits is striking. In each case they both articulate with the trapezium, 

 and digit i* has taken nearly complete possession of the distal facet of 

 the trapezoid, which is normally occupied by digit ii. The trapezoid 

 itself is narrow and smaller than the'ti-apezium ; the scaphoid in Figure 

 23 is divided into two elements, a condition which is found 07ili/ when 

 two large functional digits are added to the normal number. Other im- 

 portant facts are that digits i" and i*" are of nearly equal size, symmet- 

 trical with reference to each other, and bear hoofs which are connected 

 posteriorly by a pad of horn. 



The musculature and nerves also afford good evidence in favor of this 

 interpretation. The tendons which are normally inserted into the sec- 

 ond di(j;it ai'e wanting here. The second branch of the median nerve 

 (Fig. U^ 2), which normally supplies digit ii, still sends a large branch 

 to the radial side of digit in and may thus be identified. But dissec- 

 tions failed to disclose the small nerve which usually supplies the second 



