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The vascular action of bones, in their healthy state, is feeble, but, as in other feeble parts, it is easily excited, and disease of an obstinate and unyielding nature is apt to follow.

The morbid growths vary much in texture. The most frequent are the osseous, or those of the same structure with the original bone; but even these differ much in the density and arrangement of their particles: they have been termed exostoses. They may be of great density, and are then called the hard, or ivory; these never attain a large size, seldom exceed that of a bean, have a smooth and polished surface, and are of a flattened and hemispheroidal form, their greatest circumference being at the base; they occur in many of the bones, but generally in those of the cranium and face.

Others, being of a more loose and spongy texture, have been called cancellated. These are commonly formed by the bones of the extremities, and often attain a very considerable size; they grow from the periosteum, or from the outer surface of the bone, and are then covered by an expansion of this membrane. Sometimes they adhere by a narrow neck, and expand into a bulbous form, so that they can be very readily removed by operation, and are very apt to be broken off by external injury. Others have a broad and firmer attachment, and are of an irregular shape, often projecting in the form of a large spicula, and at other times assuming a somewhat stalactical appearance. Such frequently prove the source of much inconvenience, by impeding the motions of the muscles, or disturbing the functions of any adjoining organ. They possess no malignant disposition, but are under the same laws, though perhaps in a less degree, with their parent trunk.

On making sections of exostoses, and of the bones from which they arise, some appear to be mere enlargements or processes of the parent bone, the cancellated tissue extending itself so as to form the interior of the exostoses, whilst the exterior resembles a proportionate extension of the outer lamina. Others are evidently formed by the deposition of osseous matter externally to the outer lamina, many being dense and compact throughout, others containing an internal cancellated structure, but which is not continuous with that of the bone, as it is separated by the natural outer lamina. Their formation appears similar to that of the fœtal bones: a glutinous matter is effused, becomes dense, and is converted into cartilage; bloodvessels shoot into it, ramify throughout its whole structure in a radiated form, and deposit osseous matter. This deposit increases, and extends from a central nucleus towards the circumference; the cartilage is in part absorbed, and the new structure becomes osseous, and similar to the original tissue by the vessels of which it was formed. These tumours, even when they have attained a large size, contain a mixture of bone and cartilage, covered by a dense fibrous investment. The bony matter is arranged in spicula, adhering to the surface of the shaft or head of the bone, and projecting into the morbid mass: the spaces are filled up by a cartilaginous substance. The growth is sometimes rapid, and the tumour soon becomes troublesome from its immense size.

Frequently a bone is much enlarged throughout its whole extent, or the greater part of it, and presents a cancellated texture: sometimes, also, it is much thickened, and, at the same time, of great solidity; but such enlargements cannot be considered as tumours of bones, or exostoses, any more than those nodules of new osseous matter, which are effused in consequence of inflammation of the osseous tissue. The most frequent cause of exostoses appears to be external injury; their progress is slow, attended with slight dull pain, and often accompanied with no inconvenience; their existence can be readily ascertained, a hard and immoveable body being felt where no bone exists in the natural state of parts; but when the tumour projects into an internal cavity, the diagnosis is rendered obscure. Most frequently, they remain stationary, after having attained a certain size, and are productive of little inconvenience, the surrounding parts having accommodated themselves to the new formation. Occasionally, suppuration occurs in the soft parts, the matter comes to the surface, and a troublesome abscess is formed.

To this class of tumours would I confine the term exostosis, not including those consisting of softer materials, and possessed of a less benign action.