The five digits of the foot, in man, supported by their metatarsal bones, correspond with the digits of the hand. The great toe, which supports the chief weight of the body, has, like the thumb, two phalanges ; the other digits have three. But highly as both great toe and thumb are developed in man, they are the most inconstant elements of the extremities in Vertebrata. Dumeril, in a note appended to Cuvier's "Leçons d'Anatomie Comparée," has enunciated the law of disappearance of the digits in the different classes of animals. "That which we have seen in kangaroos authorises us in thinking that the two digits which remain in Ruminants are the third and the fourth ; thus the rule should be, that mammifers lose at first the thumb ; next the little finger ; next the index finger ; and, lastly, when there is but one complete, as in the horse, that this should be the middle finger."*
* Note by M. Dumeril in Cuvier's Traité d'Anatomie comparée, t. i. p. 534.
The homologies of the bones of the extremities have long since been recognised by Meckel in his "Descriptive Anatomy." "The bones of the upper and lower extremities correspond not only on the right and left sides, but also upward and downward; so that the lower limbs are a repetition of the upper, in respect to number and form, and the mutual relations of the different sections of which each limb is composed.