THERE is no system in the animal body to which the axiom of Guérin, viz., that “function makes the organ,” applies with greater force than to the muscular system. Every student of comparative myology knows that according to the use required of a muscle we have alterations in its volume and connections, or indeed its total disappearance, should its further services in the animal economy be dispensed with. These are the factors which render muscular homologies in many cases so difficult to determine. There is one change, however, which is much more common than is generally believed, viz., the transformation of a muscle into fibrous tissue, or, in other words, its replacement by a ligamentous structure possessing attachments similar to those of its muscular ancestor. It might almost be laid down as a law that whenever a muscle ceases to be of use for contractile purposes, and when, from its attachments, it might be of service as a ligament, that it gradually in course of time becomes transformed into fibrous tissue, and is handed down to posterity in this condition. Indeed should it merely be a case of comparative value, and should the balance of utility be in favour of a ligament, then also will this metamorphosis in all probability take place. Of all adaptations in the muscular system this is perhaps the most beautiful, and instances of it are by no means rare. Thus, in the feet of the armadillo, orycteropus, pig, walrus, and several other animals, certain of the intrinsic pedal muscles have become fibrous bands, indubitably retained for some definite purpose, although their obvious function is often obscure. The most striking examples of this, however, are to be found in the feet of the horse, ox, sheep, camel, and their allies. In these we are able not only to demonstrate with the utmost precision the particular muscles that have become ligamentous, but also the process by which the change has been brought about, and the rationale of the transformation.