110 Dr John Struthers on Variation in the Number 
man in whom the great toe and thumb are the most impor- 
tant of the digits. As the erect posture requires a great 
toe on the inner side of the foot, and a long one, the 
phalanges of the inner digit are, accordingly, developed in 
length as well as in thickness. The thumb, again, being 
better adapted for opposition by being shorter, is developed in 
thickness, and still retains its character as the shortest digit.* 
It has long been a discussion in human anatomy, whether 
the wanting bone in the thumb and great toe is a phalanx 
or a metacarpal or metatarsal bone. The view that it is the 
middle phalanx which is wanting is supported by the 
arrangement of the muscles,—(a.) by the absence of the 
tendo perforatus; (0.) by the attachment to the first phalanx 
of the short muscles which correspond to the short muscles 
of the little finger, which are attached to its first phalanx ; 
(c.) by the attachment to the metacarpal bone (with its re- 
sulting prismatic form) of the muscles which correspond to 
those which are attached to the metacarpal bone of the 
little finger; and (d.) by the position of its metacarpal bone 
in the metacarpal range. 
Opposed to this view is the one fact, that the so called 
metacarpal bone of the thumb is developed like a phalanx, 
having its epiphysis at the proximal end, while the other 
metacarpal bones have their epiphyses at the distal end. 
The discussion stood there, the difficulty being to say what 
value should be given to the developmental fact. There 
might be some special reason for the changed position of 
the epiphysis, although it would be difficult indeed to sug- 
gest any such reason, either on the longitudinal growth, or 
on the elasticity, theories of the use of epiphyses, especially 
* T have been in the habit of pointing out the interesting fact, that the 
relative length of the digits on the human hand indicates the order in which 
the digits disappear in the downward progression from the five to the one toed 
mammal,—the internal disappearing first, next the external or fifth, next the 
index or second, lastly the ring or fourth,—the extreme digit disappearing on 
alternate sides, beginning on the inner, until the middle digit alone remains 
in the foot of the horse, as demonstrated by Professor Owen in his work 
“On the Nature of Limbs.” Nor need this correspondence be regarded as 
a mere coincidence, when we consider the relative function of the digits in 
an ordinary five-toed limb. The exceptions presented to this, in the human 
foot, and in the lateral toes of the pinnigrade carnivora, are special adaptive 
modifications of certain digits. 
