Xll INTRODUCTION. 



At an early stage of development the digits may fairly be 

 looked upon as multiple organs, as they differ in no way 

 from each other; it is not therefore surprising that the 

 number should frequently vary. As development proceeds, 

 however, and each digit becomes differentiated to its 

 peculiar characters, there is apparently a stronger and 

 stronger tendency for supernumerary digits to disappear. 

 The disappearance of these abnormal digits takes place 

 according to a well-defined law, which appertains not only 

 in all such cases, but also occurs normally in the Mammalia, 

 where the typical number of digits is less than five and 

 rudiments of the absent digits remain. 



The metacarpal or metatarsal bone is the first to dis- 

 appear, and the phalanges remain loosely attached to the 

 skin and neighbouring bones by ligamentous tissue ; the 

 basal and median phalanges are lost next in order, and the 

 ungual phalanx in a rudimentary condition, or the ex- 

 tremity of the digit or nail, only remains to mark the 

 former existence of a digit. Sometimes, but very rarely, 

 a normal functional digit disappears in the same manner 

 (see No. 327). Many ruminants exhibit this condition in 

 rudimentary outer and inner digits. 



Additional digits are occasionally developed, and usually 

 exhibit the characters of the next normal digit more or less 

 closely, oftener, however, resembling that of the opposite 

 hand or foot than the corresponding digit of the same 

 limb. It occasionally, but very rarely, happens that the 

 number of digits in the opposite limb is deficient ; and 

 this has given rise to the erroneous belief in compensation 

 of growth, loi de balancement (Geoffrey), between the two 

 extremities. 



It is by no means rare in such cases for an additional 

 carpal bone to be intercalated between the normal bones ; 

 it usually resembles the trapezoid bone in the hand and the 

 second cuneiform bone in the foot. 



It is worthy of careful consideration that the addition 

 of digits is far more frequent in hoofed than in unguiculate 



