BONE. 35 



I 



Bone consists of two parts, an animal and an earthy portion, the former 

 giving toughness and the latter hardness to the osseous tissue. The animal or 

 organic part of bone may be removed by calcination, the inorganic constitu- 

 ents remaining undisturbed. After such treatment, while retaining its form, 

 the bone is fragile and easily crushed and has suffered a loss of one-third of 

 its- weight, due to the destruction and elimination of the animal matters. The 

 inorganic material, on the other hand, may be removed by the action of 

 dilute hydrochloric acid, which dissolves out the earthy matters and leaves 

 the animal part intact. Treated in this manner, the bone, although re- 

 taining perfectly its details of form, is tough and flexible, a decalcified rib or 

 fibula being readily tied into a knot. The animal constituents yield gelatin 

 after boiling with water, consisting chiefly of collagen and osseo-mucoid. 

 ' The inorganic constituents, which form approximately two-thirds of the bone, 

 include a large percentage (58) of calcium phosphate, much less calcium car- 

 bonate, with small proportions of calcium fluoride and chloride, and of the 

 salts of magnesium and sodium. 



On sawing through a bone from which the marrow and other soft parts 

 have been removed, the osseous tissue is seen to be arranged as a peripheral 

 zone of compact bone enclosing a variable amount of cancellated bone. In 

 the typical long bones, as the humerus or femur, the compact tissue almost 

 exclusively forms the tubular shaft enclosing the large marrow-cavity, while 

 the cancellated tissue constitutes the expanded extremities, with the excep- 

 tion of a narrow superficial stratum of compact bone. The irregular clefts 

 between the lamellae of the spongy bone are direct extensions of the general 

 medullary cavity and are filled with marrow-tissue. In the flat bones of the 

 skull (Fig. 48), the compact substance is arranged as an outer and an inner 

 plate, or tables, of considerable thickness, between which lies the spongy 

 bone, or diplo'e. The short and irregular bones are made up of an inner 

 mass of spongy bone covered everywhere by a shell of compact substance, 

 which often is locally thickened to insure additional strength where needed. 



The cancellated bone consists of delicate bars and lamellae united into 

 an intricate osseous reticulum well calculated to yield strength without 

 undue weight. In many positions, conspicuously in the neck of the femur, 

 the more robust lamellae are disposed according to a definite plan in order 

 to meet the strains of pressure and of tension. Although composed of the 

 same structural elements, compact and spongy bone differ in their histo* 

 logical details in consequence of the secondary modifications which take 

 place during the conversion of the spongy bone, the original form, into the 

 compact. To obtain the classic picture of bone-tissue, in order to study its 

 general arrangement where most typical, it is desirable to examine thin 

 ground sections of the compact substance cut at right angles to the axis of 

 a long bone which has been macerated and dried. 



The compact bone in such preparations, when examined under low 

 magnification (Fig. 49), is seen to consist of osseous layers arranged as 

 three chief groups: (a) circumferential lamella, which extend parallel to the 

 external and internal surfaces of the compact bone; () Havcrsian lamella, 

 which are disposed concentrically and form conspicuous annular groups, the 

 Haversian systems, enclosing the Haversian canals; and (c) interstitial or 

 ground lamella, which include the irregularly arranged tracts filling the 

 intervals between the Haversian systems and the surface lamellae. 



Each Haversian system consists of the concentrically disposed lamellae 

 and the centrally situated channel, the Haversian canal, which encloses pro- 

 longations of the marrow-tissue and ramifications of the medullary blood- 



