14 OSTEOLOGY. 



external plate, but not so thick or tough, the outer being more 

 elastic and less liable to fracture ; the connecting cancellated 

 tissue is plentifully supplied with blood-vessels. In the bones of 

 the cranium, the compact plates are called the tables, and the 

 spongy tissue between, the diploe ; the tables also in some facial 

 bones, may be widely separated, Avith air cavities between them. 

 Irregular hones include all Avhich are not classed with the 

 foregoing. They are found in the vertebral column, in the skull, 

 and also in the limbs. They usually possess many angles and 

 indentations, with surfaces for articulation and tendinous attach- 

 ment, and consist of a thin, dense, external case of compact bone 

 enclosing cancellated tissue. In proportion to their size they 

 present a much larger extent of articular surface and greater 

 mechanical strength than any other class, 



SURFACES OF BONES. 



No bone is strictly geometrical in form, although, to a casual 

 observer, some may appear so. The chief irregularities consist of 

 certain eminences and depressions, a knowledge of which is one 

 of the first requisites in the study of osteology. Eminences and 

 depressions are articular or non-articular ; the former are clothed 

 with cartilage, and assist in the formation of joints. 



Non-articular eminences exist extensively on the external 

 surfaces of most bones, and receive the attachments of tendons 

 and ligaments. They are of various shapes, sizes, and densities, 

 and frequently named from their real or supposed resemblance to 

 some known object. 



The term process or apophysis, may be generally applied to 

 prominent elevations, but processes so called are not necessarily 

 non-articular, and certain other terms are, rather arbitrarily, 

 applied. A spine is an elevation which tends to become pointed ; 

 a tubercle is a small blunt projection, which, if more developed, 

 would be called a tuberosity, while the name trochanter is 

 applied to the largest and most prominent of these. A crest or 

 ridge implies a roughened line or border. 



Non-articular depressions may serve as channels of trans- 

 mission, passing completely through a bone or part of a bone, 

 and may bear either of the names foramen, canal, aqueduct, or 

 meatus, the first name being the most used ; or they may be 

 blind cavities in the surface of the bone, and if roughened for 



