500 ON TEE LAWS 01 DIGITAL EEDUCTION. 



ANATOMY AND PHYSIOLOGY. 



ON THE LAWS OF DIGITAL REDUCTION. 

 BY JOHN A. RYDER. 



At a, recent meeting of the Philadelphia Academy I called attention to 

 several facts bearing upon an explanation of digital reduction. It was sug- 

 gested that the fact of the number of toes being least wherever mechanical 

 strains were greatest and impacts most frequent and most severe might be 

 regarded as an effect of such increased intensity of strains. To make this 

 conclusion appear valid it was only necessary to refer to the foot-structure 

 of the different orders of the class of mammals. 



It may be observed that among the primates the only creature having 

 any one toe greatly augmented in size and strength is man ; here it is the 

 great one, or the first of anatomists, its whole structure, especially the ar- 

 ticulation with the carpus, calls to mind the condition of things found to 

 e;sist in the groups which have undergone the most modification in the 

 structure of the feet, namely, the ungulates or hoofed animals, kangaroos, 

 and jumping mice. The calibre of its distal elements is greatly increased, 

 while the ento-cuneiform and jaavicular are greatly flattened or modified 

 in the same way as the magnum and unciform of the manus and the mid- 

 dle and ecto-cuneiforms of the pes are in many ungulates, or as is the cu- 

 boid in the kangaroos. 



In ungulates the third and fourth toes become functional, the second and 

 fifth either disappearing or else assuming the office of lateral supports. In 

 the jumping mice (^Dipodiclce) the second, third and fourth of the hind feet 

 are the functional ones ; in one species three toes are all that remain ; in 

 another with four the fifth, a rudimentary one, does not reach the earth ; 

 and in another species with five the first and fifth toes are rudimentary. In 

 these three animals, then, of one family and only generically separable by 

 the difference in the number of toes, we have a case in living animals re- 

 sembling the "demonstrative evidence" of Prof. Huxley drawn from fossil 

 horses' toes, which so far as the necessity for time is concerned shows that 

 creatures of almost identically the same habits and structure may be cotem- 

 poraneous, yet differing widely in the number and length of the hind toes. 

 It indicates, it seems to us, that toe modification goes on at greatly varying 

 rates. In the kangaroos the fourth and fiifth toes of the hind foot are most 

 strongly developed, while the second and third are atrophied and used only 

 to cleanse the fur. It may be noted here, also, that the toes of the fore foot 

 of the kangaroo remain entirely unmodified, and much the same as is the 

 case in the jumping mice, for the reason that the strains arc more equally 

 distributed. 



