: STRUCTURE OF BONE. 69 
able strength, equal to nearly twice that of oak, whilst it is capable of resisting a 
great crushing strain; it is stated that a cubie inch of bone will support a weight 
of over two tons. Its elasticity is remarkable, and is of the greatest service in 
enabling it to withstand the shocks to which it is so frequently subjected. In 
regions where wood is scarce the natives use the ribs of large mammals as a sub- 
stitute in the construction of their bows. Its hardness and density vary in different 
parts of the skeleton, and its permanency and durability exceed that of any other 
tissue of the body, except the enamel and dentine of the teeth. The osseous remains 
of a race over eighty centuries old is now being excavated in Egypt. 
Structure of Bone.—The structure of the bone varies with the form of the 
bone examined. If a long bone be studied in section, the shaft or diaphysis will be 
seen to be hollow, displaying a cavity of elongated shape, which contains the soft 
cellular marrow. Around this, the bone is deposited in spicules so as to form a 
loose osseous meshwork, which becomes denser as it reaches the circumference, and 
gradually merges with the compact layer which forms the outer investing envelope. 
The extremities of the bone, usually developed from separate or secondary centres 
called epiphyses, are composed of cancellous tissue, usually finer in the grain and 
not, as a rule, displaying any medullary cavity. Here the confining shell of bone 
is thin, and displays none of the stoutness which is so characteristic of the outer 
layers in the shaft. In the recent condition the extremities are cartilage-covered 
where they enter into the formation of joints. In flat bones the osseous tissue is 
disposed in two compact layers, with a layer of softer cancellous bone, here called 
the diploe, sandwiched in between. There is no medullary cavity, although in certain 
regions the substance of the diploe may be absorbed, thus forming air-spaces or air- 
sinuses. . 
True bone differs from calcified cartilage or membrane in that it not merely 
consists of the deposition of earthy salts within its matrix, but displays a definite 
arrangement of its organic and inorganic parts. Dense bone merely differs from 
loose or cancellous bone in the compactness of its tissue, the characteristic feature 
of which is the arrangement of the osseous lamellee to form what are called 
Haversian systems. These consist of a central or Haversian canal which contains 
the vessels of the bone. Around this the osseous lamelle are arranged con- 
centrically, separated here and there by interspaces called lacune, in which the 
bone corpuscles are lodged. Passing from these lacune are many fine channels 
called canaliculi. These are disposed radially to the Haversian canal, and pass 
through the osseous lamelle. They are occupied by the slender processes of the 
bone corpuscles. Each Haversian system consists of from three to ten concentric 
rings of osseous lamellee. 
In addition to the lamelle of the Haversian systems there are others which are 
termed the interstitial lamelle; these occupy the intervals between adjoiming 
Haversian systems, and consist of Haversian systems which have undergone a process 
of partial absorption. Towards the surface of the bone, and subjacent to the peri- 
osteal membrane which surrounds the shaft, there are lamelle arranged circum- 
ferentially ; these are sometimes referred to as the outer fundamental lamelle. 
The periosteal membrane which surrounds the bone, and which plays so important 
a part in its development, sends in processes through the various Haversian systems, 
which carry with them vessels and cells, thus forming an organic meshwork 
around which the earthy salts are deposited. 
The interior of the bone, viz. the marrow cavity, and the interspaces within the 
cancellous tissue, as well as some of the larger Haversian canals, are occupied by 
the marrow or medulla of the bone. This varies considerably in its composition in 
different bones. In the medullary cavity of the shafts of the long bones it consists 
mainly of fat cells, together with a few marrow cells proper, supported by a kind 
of retiform tissue, and is known as the yellow marrow. In other situations, viz. 
in the diploe of the cranial bones, in the cancellated tissue of the epiphyses of the 
long bones, the vertebrae, the sternum, and the ribs, the marrow is more fluid in its 
consistence, contains less fat, but is characterised by the presence of marrow-cells 
proper, which resemble in some respects colourless blood corpuscles. In addition 
to these, however, there are smaller reddish-coloured cells, akin to the nucleated red 
