DISEASES OF BONES. 289 



571. Structure of Bone . 



Before treating of the diseases it will be necessary to describe very 

 briefly the structure of bone. 



Bone appears to be dense and granular; but when viewed under the 

 microscope it is seen to be porous. Some parts are less, some are more 

 porous than others. The less porous part is called the Compact tissue. 

 It forms the outside of all bones. The more porous part is called the 

 Cancellated tissue. It forms the inner structure of long bones, and also 

 enters into the composition of the articular ends of all bones, and into 

 the diploe of flat bones. 



These two tissues are similar in structure. The only differences 

 between them are, that the spaces or pores in the Compact are smaller 

 and less numerous than in the Cancellated tissue ; whilst the amount of 

 solid matter is greater in the former than in the latter. The compact 

 and cancellated tissues vary in thickness in each particular bone accord- 

 ing to the strength, and other qualities required of it. In the interior of 

 all long bones is a hollow space containing the medulla or marrow. 



Though bone appears to be very dense, yet it is, even in its compact 

 tissue, sufficiently porous to admit the passage of very numerous small 

 blood-vessels through it. These vessels frequently anastomose with each 

 other. 



The outside of every bone (except at its articular extremities which 

 are tipped with cartilage) is covered with a tough fibrous membrane 

 called Periosteum, which conducts the blood-vessels into it. The inside 

 is lined with a thinner and more vascular web, called Endosteum, which 

 supports the medulla in long bones, and the cancelli in spongy bones. 



The extremities of long bones are generally expanded and roughened 

 to allow space for the attachment of muscles, ligaments, and tendons. 



572. Chemical composition of Bones. 



Bone is composed of both animal and earthy matter. The animal 

 matter consists of gelatine. It can be removed by heat. The earthy 

 constituents are phosphate of lime, which forms rather more than one 

 half of bone, and small quantities of carbonate of lime, phosphate of 

 magnesia, and fluoride of calcium. Dilute acid will remove the earthy 

 matters, and render the bone flexible and soft. 



573. Nutrition of Bone. 



The nutrition of bone is derived from three sets of arteries. Large 

 arteries enter about the centre of long bones, whilst smaller enter at the 

 articular ends, and very minute vessels enter on the periosteal membrane. 



574. Development of Bone. 



Bone has three stages of development, namely the gelatinous, the 

 cartilaginous, and the osseous. In the earliest stage of foetal life 

 the germ of bone is entirely gelatinous. This is succeeded by temporary 



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