188 
Proceedings of Eoyal Society of Edinburgh. [sess. 
Each metacarpal and metatarsal consisted of an ossified shaft and of 
cartilaginous extremities ; the proximal piece of cartilage measured 
from 4 to 5 mm., the distal from IJ to 2 mm. The phalangeal 
part consisted of an elongated piece of cartilage representing the 
first and second phalanges (1, 2, Fig. II.), and of a small terminal 
portion representing the third phalanx (3, Eig. II.). The large 
piece was articulated to the slightly-rounded end of the epiphysis 
by an imperfectly-formed joint. The joint between the second and 
third phalanges looked as complete as the basal joint, hut, as 
sections showed, this was not the case. In some sections the 
epiphysis of the metacarpal was completely separated by a distinct 
gap from the first phalanx, hut, in the case of the second and 
third phalanges, though there was a deep cleft extending well 
across between them, the separation was never complete. The 
formation of a joint between the first and second phalanges in the 
specimens examined had either never been attempted or it had been 
arrested at the initial stages. At the most, there was a shallow 
cleft on what appeared to he the anterior or extensor aspect.'^ 
Having once recognised that there is a piece of cartilage beyond, 
and quite distinct from, the epiphysis at the distal end of the meta- 
carpals and metatarsals, there is never any difficulty in making out 
the metacarpo-phalangeal and metatarso-phalangeal joints. But in 
most cases it is extremely difficult to define the phalanges, more 
* The practical absence of a joint between the first and second phalanges is 
extremely interesting. Granting that the effects of disuse are not transmitted, 
it is all but impossible to account for the gradual reduction of digits generation 
after generation, century after century. Anything that throws light on the 
'modus operandi of reduction is therefore well worth recording. In man the 
little toe is slowdy changing from a three-jointed to a two-jointed toe. This 
degeneration has often been ascribed to boot-pressure, and has been again and 
again brought forward as an example of the transmission of acquired characters. 
The condition of the second and fourth digits in the embryo horse, considered 
along with the fusion of the first and second phalanges of a “restored” second 
' digit of a foal in my collection, may be held to prove that degeneration of the 
digits is, apart from any external influence, accompanied by an arrest in the 
formation of the phalangeal joints. External pressure could not, of course, be 
a factor in the degenerative process in the case of the horse. Hence the reduc- 
tion of a digit may be said to consist not only of an arrest in the growth of the 
phalanges but also of an arrest in the formation of the joints between them — 
not, as might have been expected, by the disappearance, one after another, of 
the phalanges from below upwards. Why this arrest should take place in some 
digits and not in others, and proceed at an increasing rate generation after 
generation, is, as already indicated, extremely difficult to explain. 
