By the term exostosis should be understood a purely bony mass, set upon a bone forming with it an organic whole, and, where it is possible, originating, or proceeding, from the bone. When its development is complete, and often at the beginning of its growth, its texture is always homologous with that of its base and point of origin, whether that be compact or spongy. Hence all new growths upon or within a bone, which hold any other relation to it, are excluded, although they be composed more or less of normal bony texture, and even although composed of such texture altogether. Bony growths, however, which proceed from the periosteum, but sooner or later become united with the bone, are admitted.

The most important varieties in an exostosis relate to its texture, to the point from which it originated (though in many cases this cannot be determined with certainty), and to the mode of its development. To these varieties the sundry and wide differences in its size, form, mode of attachment, etc, are mainly referable.

Exostoses are composed sometimes of compact, sometimes of spongy bony substance; and although some are made up of both these substances, yet the division into those which are compact and those which are spongy is so far valuable, that it expresses their original condition and their development.

a. The most frequent of the exostoses is the compact. It occurs on compact bones and parts of bones, particularly on the outer table of the cranial bones.

It appears as if it had been planted on the surface of the bone from without: in general it is a plane convex nodule, the margin of which is abrupt, and often separated from the bone beneath by a furrow. This furrow is generally narrow, sometimes being but just perceptible, and about the thickness of a hair; but frequently it is deep, and forms a fissure between the tumor and its basement. It gives the exostosis the appearance of having been glued on, or of sitting, mushroom-like, on a very short stalk.

Not only is this exostosis in all cases compact, but it often exceeds in density the bone from which it springs: it is then known as the ivory exostosis. It is especially liable to be formed on bones which are themselves indurated.

It is compact from the very first; and grows in such a way that the layers which are added to it always at once become as dense as ivory. Neither the most superficial and most recent strata, nor the smallest of those exostoses, which form near larger ones, even though no larger than hemp-seed or a lentil, is ever seen to contain any spongy structure. New layers and old, large exostoses and small, are equally dense and hard. When they are minutely examined, the number of peripheral lamellae is found to be very considerable; and the corpuscles lying amongst them are long. The Haversian canals are small and far apart, and many of them are surrounded by a distinct and completely defined (völlig abgeschlossenen) lamellar system. With regard to the corpuscles, we find large tracts without any of them, while at other spots they are clustered together in dense groups.

The number of these exostoses occurring in one person, and even on the same bone, is sometimes very considerable; especially if the very small ones, which are easily overlooked, be also enumerated. I have met with them almost exclusively on the skull, where, like induration, they do, in fact, most frequently occur; but they are likewise observed on the long bones, and on the bones of the pelvis.

They vary in size from that of a flattened hemp-seed or a lentil, which is scarcely perceptible, to that of a walnut, or a hen's egg, and even to greater dimensions. Their most common size ranges between that of a pea and that of a hazel-nut.

While their usual form is that of a plane convex nodule, their surface, whether even or uneven, is always smooth and polished. If they grow beyond the ordinary size, they become round, or oval, or, as they generally rather increase in length, they form a more or less cylindrical, horn-like projection. There is another form which occurs with them on the inner table of the skull near the frontal crest: it has a peculiar humifuse1 character, or the appearance of a convoluted wreath.

The color of these exostoses is white or yellowish-white, - whiter than that of the bone to which they are attached.

As we cannot associate exostoses, in respect to their cause, with the various inequalities and nodules that occur on bones from constitutional disease, especially from syphilis, the occasion of their origin must be said to be unknown. As I have already remarked, they are very generally found on the bones of the skull, of which one or more are at the same time the seat of induration, and not unfrequently bony formations are found also on the dura mater.

1 [" Humifuse" - growing parallel to the surface, but attached only at its point of origin-like the sterns of plants which creep along the ground without taking root. (Palmer's Pent. Dict.) - Ed].