(From out of, and a bone).

Hyperostosis, a tumour on a bone. Mr. Pott calls it an enlargement of the bone. Its hardness equals, or rather exceeds, that of the bone from which it proceeds. Mons. Petit calls the spina ventosa by the name of exostosis, but the disorders are very different. Dr. Cullen places this disease in the class locales, and order tumores, which he defines a hard tumour forming in the bone.

The exostosis is caused by a discharge of a superfluous quantity of ossific matter upon the part where it is seated, or from a separation of the bony lamellae. The cause of each is an irritation arising from some degree of inflammation, often the effect of syphilitic virus. If from disease, the chief cause must be removed. Whatever, however, is called a venereal exostosis, is only an enlargement, or rather a thickening, of the periosteum. It is sometimes very painful, and should be distinguished from the rickets, which affect the ends of the bones chiefly; while an exostosis is rather in the middle of the long bones. The rickets are also sufficiently distinguished by the generally diseased habit. If no general disease exists, the patient may live to advanced age without any considerable inconvenience.

When the nature of the cause is understood, and we have reason to hope for success, the bone should be laid bare, and the diseased part taken away with a chisel. This will succeed if the habit is not much diseased; but if the constitution is also faulty, and the exostosis proceeds from the exuberance of bony matter, amputation is the only method of relief; though generally the case is most safely left to nature.

Sometimes a preternatural hardness of the ligament is called an exostosis; this spurious sort, as well as the venereal nodes, is relieved by mercurials.

Exostosis happening in the middle of hard bones are generally hard in all their parts; but those near the ends, or about the joints, have often only a hard external lamina. When this disorder happens on the bones within the skull, the consequence may be an apoplexy, epilepsy, or a palsy. See Petit's Diseases of the Bones, part ii. chap. xvi. Bell's Surgery, vol. v. p. 541.