Page:Popular Science Monthly Volume 42.djvu/701

Rh For the first week or more this exudation of reparative material takes place at the point of fracture—occurring externally between the periosteum and the bone, and internally between the medullary membrane and the bone. At the end of about three weeks this provisional callus acquires a firmness about equal to that of cartilage; which, at the end of three months, both internally and externally, ossifies—the ossification being more or less of a spongy nature. Shrinkage now takes place, and this spongy ossification becomes modeled down, forming compact bone. Still, at this stage the fractured ends are only united by fibrous tissue, though the surrounding ossified callus holds them firmly together.

Finally, this provisional callus undergoes more or less complete absorption, and the permanent callus forms directly between the fractured ends of the bone. This results in the nearly complete disappearance of the periosteal enlargement, and, internally, in the re-establishment of the medullary canal. Normally, this is the course of the union of fractures in long bones, of the character referred to above, but there are a number of exceptions to it, and irregular unions occur which do not require comment from me in the present connection.

Several years ago I obtained a turkey vulture (Cathartes aura) that had survived a glance shot from a carbine ball which fractured both bones of the left forearm of the upper extremity. It had also sustained other fractures, all of which I fully described in the New York Medical Journal (see No. 3 of foot-notes given antea). This bird I subsequently killed with chloroform and prepared its skeleton. Upon examination it was discovered that the left ulna and radius were each fractured at the points shown in the accompanying cut (Fig. 1), and were at the time of its death at that stage of union where the provisional callus is well under way toward absorption.

From my various observations in such cases I have arrived at the conclusion that at this stage the weakest point in the provisional callus lies in the plane of the meeting of that material, as it is furnished by the two broken ends of the bone—in other words, it is at its thickest part and in the plane of the fracture. I mention this fact, as reference will soon be made to it again, further on.

Owing to the support afforded by the quill-butts of the secondary feathers of the wing in a bird, acting as a compound splint of Nature's furnishing, the radius and ulna in that class of vertebrates