182 HISTORY OF THE HUMAN BODY 



9%), a derivative, sometimes of the first, sometimes of the 

 second, metatarsal. 



The typical number of digits is five, but this number is fre- 

 quently reduced by the loss of digits at either end of the series. 

 Instances of reduction, usually with vestiges of the missing 

 digits, are of frequent occurrence and range from cases with 

 the loss of the first alone, as in certain salamanders, or of the 

 last alone, as in the feet of birds, to the extreme case ex- 

 hibited by the horse, in which the middle digit is alone de- 

 veloped, accompanied by vestiges of II and IV. 



In the pig and the ox two digits, II and V, considerably 

 reduced in size, are set behind the two well developed ones, 

 III and IV, and terminate in horny spurs that do not touch the 

 ground. Digit I is entirely wanting. 



The occasional occurrence of hyperdactylism, or cases with 

 supernumerary digits, in all groups of tetrapod vertebrates, 

 together with the presence, in numerous cases, of extra bones, 

 sesamoid and otherwise, beyond and at the sides of the true 

 digits (e.g., os falciforme in Fig. 48, e), have often been in- 

 terpreted as pointing to a previous condition with more than 

 five digits, a condition that would well accord with the de- 

 rivation of the hand from a fin, since in most cases the latter 

 structures possess more than five fin-rays. These phenomena 

 are not, however, so interpreted by all morphologists, and the 

 subject is a controversial one at present. Such an hypotheti- 

 cal digit placed before the thumb or great toe is called a prce- 

 pollex or prce-hallux respectively; the one continuing the 

 series beyond the fifth is a post-minimus. 



The chiridial appendage just described, although it forms 

 the universal plan upon which is based that of all tetrapod 

 vertebrates, is yet capable of great modification and has en- 

 abled its various possessors to adapt it to a great variety of 

 uses. The detailed consideration of this belongs to the field 

 of special comparative anatomy and does not come within the 

 scope of this, work, yet as an illustration of the general prin- 

 ciple of adaptation a few cases may be mentioned which show 

 certain of the modifications acquired in the successful adap- 



