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The Second and Fourth Digits in the Horse: their Development and Subsequent Degeneration

Published online by Cambridge University Press:  15 September 2014

J. C. Ewart
J. C. Ewart
Affiliation:
Regius Professor of Natural History, University of Edinburgh
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Extract

During the last fifty years not a few familiar and apparently quite uninteresting structures have all at once assumed unusual importance and arrested the attention of naturalists in all parts of the world. This is especially true of what used to be known as rudiments, of what we nowadays generally designate vestiges. Of all the known vestiges, the most familiar are perhaps the “splint” bones of the horse. For long the interest in these splints was extremely limited, but since it was shown that they correspond to the functional second and fourth metacarpals and metatarsals of other vertebrates they have attracted the notice of both comparative anatomists and palæozoologists, and are now invariably looked upon as affording strong evidence in support of the view that the horse has descended from polydaetylous ancestors.

Type
Proceedings
Information
Proceedings of the Royal Society of Edinburgh , Volume 20 , 1895 , pp. 185 - 191
Copyright
Copyright © Royal Society of Edinburgh 1895

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References

page 186 note * These cells probably represent an articular disc.

page 188 note * 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 slowly 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 reduction 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.

page 189 note * I may mention that on removing the investing tissues I noticed what looked extremely like vestiges of the flexor tendons.

page 189 note † Ewart, Jour, of Anat. and Phys., Jannary 1894.

page 190 note * Journ. of Anat. and Physiol., vol. xxiii. p. 507.