CHAPTER VII. 



THE STRUCTURE AM) COMPOSITION OF BONK 

 JOINTS. 



Gross Structure of the Bones. The bones of the Body 

 have all a similar structure and composition, but on ac- 

 count of differences in shape they are divided by anato- 

 mists 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) Ir- 

 regular 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 two 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 se- 

 lect the humerus. Externally in the fresh state it is covered 

 by a dense white fibrous membrane very closely adherent to 

 it and containing a good many small blood-vessels. 

 This membrane is called 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 nu- 

 trition of the bone, which dies if it is 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 in contact with other bones and glide over them 



