INTRODUCTORY. 9 



Flat bones have one axis mucli shorter than the others, 

 whence the bone assumes an expanded form and serves for 

 the attachment of large muscles or for forming the walls of 

 cavities. They present no medullary canal, but between 

 the outer compact layers is cancellated substance, which in 

 the cranial bones is termed diploe. Examples of flat bones 

 — scapula, parietal bones. They have a centrally situated 

 centre of ossification, with others for their processes, and 

 their Haversian canals are perpendicular to their flat sur- 

 faces. Some parts of them seem to result from ossific 

 deposit in fibrous membrane, and not from change of car- 

 tilage. 



Irregular bones are those in which no marked difference 

 exists between the length of their axes. They consist of 

 cancellated structure surrounded by a thin layer of com- 

 pact substance. The centres of ossification are related to 

 the peculiarities of each bone. Examples— petrous, temporal 

 bones, os pedis, &c. In this class are included " short hones." 



Bones derive their supply of blood from the arteries which 

 run nearest to them ; they are surrounded by a strong 

 membrane composed of white fibrous tissue, periosteum, 

 which is very vascular, and from which minute vessels pass 

 to foramina on the surface of the bone in connection with 

 the Haversian canals. In addition to this long bones 

 generally present a small opening {foramen medullare), 

 which pierces the shaft of the bone, and through which 

 passes a branch from the nearest important artery which 

 runs to the medullary canal, and breaks up to supply the 

 medulla and to send small branches, corresponding with 

 those from the periosteum, into the bone ; and a third 

 source of supply is derived from small articulatory foramina 

 surrounding those surfaces by which these bones come into 

 indirect contact with each other. These articulatory sur- 

 faces being covered with cartilage, are not covered by 

 periosteum, which also is absent from some places where 

 tendons become directly attached to the bone, and from 

 bones, like the ethmoidal cells and the ossa turbinata, which 

 are intimately clothed with highly vascular mucous mem- 

 brane. Bones are but slightly supplied with nerve- 

 force, their lymphatics have not been satisfactorily de- 

 monstrated. Bones present eminences and cavities, either 

 of which may be articulatory or no n- articulatory. 



