290 CHAPTER 35. 



cartilage, and subsequently bone is gradually deposited in the meshes of 

 the cartilage, and this latter tissue is then absorbed. The points at 

 which bone is deposited are called centres of ossification. Three of 

 these centres are usually found in each bone, viz. one for the shaft and 

 one for each extremity. 



575. Periosteum. 



Periosteum, or the outer covering of bone is composed of dense, tough, 

 inelastic, vascular, white fibrous tissue. Its function is to nourish, 

 strengthen, and protect the bone. It also serves as a medium to conduct 

 the blood-vessels over the surface of the bone. The periosteum is 

 exceedingly strong, so strong and tough, that in cases of fracture of a 

 bone, it is often enabled to hold together the two parts. 



Periostitis or inflammation of the periosteum, may be readily caused 

 by injuries, by exposure to cold, by blood diseases, or by extension of 

 inflammation existing in the bone. The secretion effused from the 

 periosteum during the process of inflammation is very apt to solidify, 

 and in due time to become bone. Hence, we often meet with exostoses 

 as a result of inflammation of the periosteum, or of any of the osseous 

 structures. The great pain produced by inflammation of a bone is due 

 to the very inelastic nature of its outer covering. Nerves are freely 

 distributed to the periosteum, and accompany the arteries into the 

 interior of the bone. 



576. Caries. 



Caries of bone is analogous to ulceration of soft structures, and is the 

 result of a slow inflammatory process causing softening and suppuration. 



Caries generally affects the cancellated structure of bones, but the rule 

 is by no means absolute, as the disease frequently exists in the compact 

 tissue. 



The primary symptom is apparent swelling of the bone. After a time 

 sinuses form, from which an offensive sanious discharge exudes. If a 

 probe be introduced into one of the sinuses, the interior of the bone will 

 be felt to be soft. As the disease progresses, the part affected assumes a 

 worm-eaten, excavated, and broken-up appearance. The many cavities 

 become the seat of various exudations. The cancellated tissue gradually 

 breaks down, and is discharged in minute fragments, along with pus. 



The exudation proceeding from, caries is distinguished by being thin, 

 acrid, excoriating, sanious, and foul on account of its consisting of the 

 decomposing organic materials of bone. It does not, however, always 

 appear to be thin, because when it afterwards, as is sometimes the case, 

 flows over a mucous membrane, it may become mixed with the secretion 

 of that tissue, and in consequence may assume a clotted appearance. 



The periosteum is injuriously affected by the acridity of the discharge, 

 and after a time is destroyed by it. When this result has been produced, 

 the disease proceeds more rapidly, because the carious portion of the bone 

 is now deprived of the nutriment which ought to be supplied to it by the 



