BONE is the hard malerial of the skeletons or frameworks of mammalian anon:11s. reptiles, and birds. In its earliest stases, it is termed tempontry cartilage (q.v.), and consists of cells massed together, except in the flat bones, as those of the skull and shoulder-blade, of which the primary foundations are to a great extent of fibrous tissue. Points or centers of ossification fitnii, the cells alter their form and arrangement, and a deposit of earthy materials, phosphate and carbonate of lime, takes place, rendering, the former flexible substance rigid. By soaking a B. in a dilute mineral acid. we can dissolve these earthy matters, and render tragain flexible; on the other hand, if we expose it to intense beet, the animal matter (gelatine) is got rid of, and though the bone retains at first its form, the slightest totieh will cause its now unsupported earthy mat ter to ertunlile away. We see., in the ill-nou•isbed children of large towns, too' many exts!uples of bow necessary a proper relation of these Iwo elements of 13. to each other is; in the disease tidied tickets, the earthy matter is deficient, and the too flexible leg-bones bend under the weight of the trunk. In the aged persou, again, the 13. sub stance becomes more densely packed with earthy matter, and becomes brittle, rendering them peculiarly liable to fractures.
The bones of the skeleton are classified acconling to their shapes—viz., as long bones, e.g., the thigh•lione and ann-lione; flat bones, as the shoulder-blade and skull bones; short and irregular bones, as those of the wrist or vertehree,• The substance of bones is arranged differently in different parts—either hard and close. which is called the condensed substance, or loose and reticulated, called the cane-dialed structure. Long tortes have a shaft of hard substance terminating at each end in soft or cancellated structure; in the hitter :situations, the 13. is more expanded and rounded off to enter into the formation of s joint. Irrerpdar bows consist of a shell of condensed tissue, incles ing, a mass of cancellated structure, and are smoothed off into surfaces adapted to those of the adjoining, bones. Alt bones consist of two layerrs of hard tissue, with an intermedi ate cancellated structtire. Anatomists also talk of mt:red bones, those which are both long and flat, as the ribs, the 1in.:1st-hone, and the lower jaw.
'file shaft of :t long 13. is hollow, and filled with an oily substance. the marrow (q.v.):
the space in whieli the marrow lies is called the medullary canal. This fatty substance is also found in the cancellated structure of short and mixed, and in the diplot< of flat bolis, and eveu in the condensed tissue. Bones are covered externally by periosteum (q.v.), and on the surfaces of the cavities within by a fine membrane called interim] periosteum or medullary membrane. 13. is largely supplied with blood-ve.ssels, which are continued into it front those of the periostemn; the largest are those which enter the ra eancellated ends of the long bones. The medullary membne receives a special artery for the supply of thy compact tissue next the canal. This vessel enters the bone gen cnilly rather above its middle, and divides into two branches, one of which tins up, the oilier downwards, both dividing into numerous branches, anastomosing with the vessels we have alluded to as citerinr the caneellated tissue, After the arteries enter the com pact tissue of bones, they rim in small capillary canals, invisible to the caked eye. which' permeate the bone, and antistomose, leaving oblong loops or meshes, :I he reins of 13. arc also contained in these canals, but they are larger than the artenes. and possess at irregoiar intervals, where branches meet, dilatations or reservoirs for the blood.
These canals, named Haversian. after their discoverer. (lepton Havers. an old English anatomist, vary in diameter from to of an in. They -hike a longitu dinal direction, and if a tnaisverae section is examined wider the microscope, it appears pierced with holes, which are the Haversian canals cut across. Each canal' is sur rounded by its own layers of condensed structure, forming in the aggregate a hollow rod or pin, called the Elaversian system, running through the plates of which the B. is composed, and securing their cohesion, In addition to these, there are to be seen a number of minute spaces or lacuna, generally oval, in man; from these pass numerous pores or eanalieuli, which are directed to the Dearest vessels: those in the periosteal, or outer lamella, pugs into the B. front orifices on its surface, and the lacunic face outwards. The pores of the internal layer open on the medullary canal, and its lacunae face towards it, and time leen= in the layers Around each Haversian canal face towards, and their pores open into it.