150 DISEASES OF THE BONES AND ARTICULATIONS. 



composed of cancelli, with a very thin layer of corapact tissue. 

 Whilst they are of greater circumference, their weight is not 

 relatively greater than the shaft, their increase being due to 

 an expansion of substance. This arrangement lightens and 

 strengtliens the bone, and modifies inflammatory action, which 

 consequently partakes more of the nature of that of the soft 

 tissues. 



The hardest part of tlie bone is a thin laminal layer, lying 

 next to the articular cartilage. It is non- vascular, insensible, 

 and destitute of Haversian canals, and, in inflammatory disease 

 of the articulations, it is affected in a remarkable way, which 

 will be referred to hereafter. 



Each long bone has a canal in the centre of its shaft, called 

 the medullary canal, which is not continued to the extremities. 

 It contains medulla or marrow, and is lined by a delicate 

 vascular membrane, the endosteum, which is prolonged into 

 the cancellated structure and Haversian canals. It is supplied 

 with blood by the nutrient arteries, which anastomose with 

 those supplied by the periosteum. 



The periosteum invests the whole bone, except its articular 

 extremities. It is a complex structure, consisting of a fibrous 

 membrane, and a lining of germinal or nucleated membrane ; 

 both of which are continued into the Haversian canals. This 

 inner lining assists in the formation of new bone, but the 

 fibrous portion has no formative power. 



Many experiments have been made to prove that the 

 periosteum possessed this formative power, but they are stiU 

 inconclusive ; and the only practical deduction obtained is, 

 that when it is destroyed, there is a want of nutrition and 

 reproductive power, in proportion to the destruction of the 

 formative membrane and the vessels passing along with it to 

 the substance of the bone. 



The presence of this formative membrane, internal to the 

 fibrous coat, will at once explain how subperiosteal exudation, 

 as in the case of splints and other exostoses, is converted into 

 bony structure. 



Having in view aU the peculiarities of structure found in 

 the various bones, we will now consider the diseases aff^ecting 

 them. 



