The Spongy Exostosis proceeds from a circumscribed rarefaction, or expansion, of the bony tissue (osteoporosis); it forms a tumor of cellular texture abounding with marrow, which is surrounded by a compact layer or rind. It is sometimes developed from compact bony tissue, sometimes from spongy substance, and either from the peripheral laminae of the bone, or from its interior. It presents, accordingly, many striking varieties of external form and of internal structure. Its rind or external layer unites with that of the bone; its surface is uneven.

Sometimes it forms a slight, rounded elevation, above the surface of the bone, sometimes a more sharply circumscribed, hemispherical tumor; or it has a still narrower base, and is globular.

We not unfrequently find, near or upon the articular ends of long bones, and especially on the tibia and femur towards the knee-joint, a rounded, gnarled, and uneven excrescence, sometimes lobulated, or branched, and set upon a well-formed stalk; sometimes it has the form of rounded or angular, thorn-like processes. Such excrescences may be occasionally found near the articulations on most, if not all, the long bones of a skeleton.

Lastly, there is another form of exostosis allied to the spongy form, which has not only a spongy texture within its compact rind, but also a well-formed medullary cavity communicating with the medullary tube and cells of the bone; and thus presenting, as it were, a dilatation of the medullary cavity beyond the surface of the bone. Whether such really be the mode of origin of this form of exostosis, and a central cavity of this kind, communicating with the medullary tube of the bone, really exist from the commencement of the disease (in which case this exostosis would clearly rank with the spongy form), or whether the central cavity be formed in it subsequently, as it is in callus, is as yet unknown.

In the skull, the expansion of the diploetic structure sometimes distends both the compact tables, and thus there is an internal exostosis corresponding with the external.

The spongy exostosis continues for an indefinite period in its original spongy state: not unfrequently perhaps it may remain so permanently; but more commonly new substance is deposited in its interior, and more or less induration ensues. The compact exterior wall acquires considerable thickness, and encloses a mass of cancellous substance, or a well-formed central cavity: sometimes spots in its interior are found compact too, or it may even become uniformly solid throughout.

In some parts, and even in the whole of the exostosis, a renewal of rarefaction, or osteoporosis, very often appears to succeed this process of condensation. It may be in this manner that the growth of the spongy exostosis from within outwards is chiefly effected.

They very often grow to a considerable size.

Exostoses are formed, as a general rule, in the outer layers of a bone, and they grow and project outwards: but, in a few cases, they have been seen to advance in the opposite direction, and protrude within the medullary canal. They are then named enostoses, - a name which we are accustomed to give to exostoses which project into a cavity, such as the skull, the orbit, or the pelvis. Sometimes both are met with together. An exostosis which encircles a cylindrical bone more or less completely, is called periostosis; and so on.

The cause of exostosis is not yet ascertained. It occurs, but not constantly, under certain local circumstances, as after a blow or a fracture; in the latter instance, it is merely exuberant callus; and no definite and clearer cause for it can be assigned. In most cases the periosteum covering the exostosis is in its natural condition; sometimes it is thicker than natural, and hypertrophied, and adheres with unusual firmness.

Exostoses are found in every period of life; those of the spongy kind occur even in children and new-born infants.

Usually, when they have reached a certain bulk, they continue for the remainder of life unchanged. Sometimes, and even in cases of the ivory exostosis, they have been observed to diminish in size, either by absorption, or, as it were, by contracting, while, at the same time, their structure increases in density. Spongy exostoses sometimes become carious and are destroyed; while, in a few cases, the ivory exostosis appears to have been attacked with necrosis, and thrown off.

The callus deposited around fractures frequently resembles the exostosis; and one form of the osteophyte resembles it still more: it is that which is occasioned by a circumscribed chronic inflammation of the outer layer of a bone, and which finally becomes condensed (sclerosed), and adheres to its surface. Lastly, there is one form of bony growth yet to mention, which is produced at first by the dura mater or the periosteum on the side next the skull, and afterwards unites with the bone. It is generally flat, and the surface by which it adheres to the periosteum is rough: it is most frequently found on the inner table of the vault of the skull, where, as a product of the dura mater, it fits into the depressions of the vitreous table, and becomes firmly soldered on.