CHAPTER VII. 

 THE STRUCTURE AND COMPOSITION OF BONE. JOINTS. 



Gross Structure of the Bones. The bones of the Body 

 have all a similar structure and composition, but on account 

 of differences in shape they are divided by anatomists into 

 the following groups: (1) Long bones, more or less cylindrical 

 in form, like the bones of the thigh and arm, leg and forearm, 

 metacarpus, metatarsus, fingers and toes. (2) Tabular bones, 

 in the form of expanded plates, like the bones on the roof and 

 sides of the skull, and the shoulder-blades. (3) Short bones, 

 rounded or angular in form and not much greater in one 

 diameter than in another, like the bones of the tarsus and 

 carpus. (4) Irregular bones, including all which do not fit 

 well into any of the preceding groups, and commonly lying 

 in the middle line of the Body and divisible into similar 

 halves, as the vertebrae. Living bones have a bluish-white 

 color and possess considerable elasticity, which is best seen in 

 long slender bones such as the ribs. 



To get a general idea of the structure of a bone, we may 

 select the humerus for examination. Externally in the fresh 

 state it is covered by a dense white fibrous membrane very 

 closely adherent to it and containing many small blood-vessels. 

 This membrane is the periosteum ; on its under side new 

 osseous tissue is formed while the bone is still growing, and 

 all through life it is concerned in maintaining the nutrition 

 of the bone, which dies if it be stripped off. The periosteum 

 covers the whole surface of the bone except its ends in the 

 elbow and shoulder joints; the surfaces there which come into 

 contact with other bones and glide over them in the move- 

 ments of the joint have no periosteum, but are covered by 

 a thin layer of gristle, known as the articular cartilage. Very 

 early in the development of the Body the bone in fact -was 

 represented entirely by cartilage; but afterwards nearly all 

 this was replaced by osseous tissue, leaving only a thin car- 

 tilaginous layer at the ends. 



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