2 STRUCTURE OF BONE. 



Bones are divisible into three classes -.Long, flat, and irregular. 



The Long bones are found principally in the limbs, and consist of a 

 shaft and two extremities. The shaft is cylindrical or prismoid in 

 form, dense and hard in texture, and hollowed in the interior into a 

 medullary canal. The extremities are broad and expanded, to articu- 

 late with adjoining bones ; and cellular or cancellous in internal struc- 

 ture. Upon the exterior of the bone are processes and rough surfaces 

 for the attachment of muscles, and foramina for the transmission of 

 vessels and nerves. The character of long bones is, therefore, their 

 general type of structure and their divisibility into a central portion 

 and extremities, and not so much their length ; for there are some 

 long bones, as the second phalanges of the toes, which are less than 

 a quarter of an inch in length, and almost equal, and in some instances 

 exceed, in breadth their longitudinal axis. The long bones are, the 

 clavicle, humerus, radius and ulna, femur, tibia and fibula, metacarpal 

 bones, metatarsal, phalanges and ribs. 



Flat bones are composed of two layers of dense bone with an inter- 

 mediate cellular structure, and are divisible into surfaces, borders, 

 angles, and processes. They are adapted to enclose cavities ; have 

 processes upon their surface for the attachment of muscles ; and are 

 perforated by foramina, for the passage of nutrient vessels to their 

 cells, and for the transmission of vessels and nerves. They articulate 

 with long bones by means of smooth surfaces plated with cartilage, and 

 with each other either by fibrous tissue, as at the symphysis pubis ; 

 or by suture, as in the bones of the skull. The two condensed layers 

 of the bones of the skull are named, tables ; and the intermediate 

 cellular structure, diploe. The flat bones are the occipital, parietal, 

 frontal, nasal, lachrymal, vomer, sternum, scapulae, and ossa inno- 

 minata. 



The Irregular bones include all that remain after the long and the 

 flat bones have been selected. They are essentially irregular in their 

 form, in some parts flat, in others short and thick. In preceding edi- 

 tions of this work the short and thick bones were made a separate 

 class under the name of short bones. This subdivision has been found 

 to be disadvantageous, besides being arbitrary, and is, therefore, now 

 omitted. Irregular bones are constructed on the same general princi- 

 ples with other bones ; they have an exterior dense, and an interior 

 more or less cellular. The bones of this class are, the temporal, sphe- 

 noid, ethmoid, superior maxillary, inferior maxillary, palate, inferior 

 turbinated, hyoid, vertebrae, sacrum, coccyx, carpal and tarsal bones, 

 and sesamoid bones, including the patellae. 



Structure of Bone. Bone is a dense, compact, and homogeneous sub- 

 stance (basis substance) filled with minute cells, (corpuscles of Purk- 

 inje) which are scattered numerously through its structure. The basis 

 substance of bone is subfibrous and obscurely lamellated, the lamellae 

 being concentric in long and parallel in flat bones ; it is traversed in 

 all directions, but especially in the longitudinal axis, by branching and 

 inosculating canals (Haversian canals) which give passage to vessels 



