384 GENERAL ANATOMY. 



sembles the natural bone, in its periosteum, its medullary 

 spongy cavities, its texture, its semi-transparency, and its che- 

 mical composition; but this perfect production is of rare oc- 

 currence. It has been met with under the form of an isolated 

 body in the dura mater. I have also seen it, but almost en- 

 tirely compact, under the form of laminae, situated in the an- 

 terior vertebral ligament. The bony plates which cover the 

 costal cartilages are of the same nature. There is also some- 

 times observed a perfect, but compact, ossification, under the 

 form of the hydatiferous cyst. 



Accidental ossification, which also presents several varieties, 

 is often an effect of age. Many old persons, however, are not 

 affected with it. Its causes are most commonly irritation 

 and chronic or latent inflammation. It is more frequent in 

 cold than in warm countries. It commences with a plastic 

 production, and sometimes passes through the semi-cartilagin- 

 ous or fibrous states, but at other times does not. In general, 

 it produces no inconvenience except by its bulk or mechanical 

 effects. 



The transformation of permanent cartilages into bone may 

 be regarded as intermediate between natural and accidental 

 ossification. 



606. Exostosis* is also an accidental bony production, 

 sometimes perfect, and often stony, and resembling ivory. 

 The periosteum being irritated or inflamed, there takes place, 

 at its inner surface, in its substance, and in a part of greater 

 or less extent of its breadth, a deposition of soft, organizable 

 matter; it constitutes periostosis, which terminates variously. 

 In many cases it ossifies, constituting at first a kind of epiphy- 

 sis or bone, distinct and separable from the natural bone, to 

 which the exostosis is at length firmly attached. Sometimes it 

 consists of a very circumscribed nodus, which has been rapid- 

 ly developed. At other times it forms slowly, and consists 

 of a voluminous and foliated mass. Sometimes, also, a whole 

 limb, or even a larger portion of the skeleton is affected by it. 



Spina-ventosa, in place of always consisting of a morbid 



* On Exostosis, by M. A. Cooper, in Surgical Essays, Part I. Lond. 1818. 



