84 OSTEOLOGY. 





A better idea of the disposition of the bony framework of a long bone can be 

 obtained by the examination of a section of a macerated specimen. In such a 

 specimen the marrow has been destroyed and the osseous architecture of the bone 

 is consequently better displayed. 



Within the body of the bone is seen the marrow cavity extending towards, but not 

 reaching, either extremity of the bone. This cavity is surrounded on all sides by 

 a loose spicular network of bone, which gradually increases in compactness until it 

 reaches the circumference of the shaft, where it forms a dense surrounding wall. In 

 the shaft of a long bone the thickness of this outer layer is not the same throughout, 

 but tends to diminish as we approach the extremities, nor is it of uniform thickness 

 on all sides of the bone. All the long bones display curves in varying degree, and 

 it is a uniform rule that the thicker dense bone is found along the concave surface 

 of the curve, thus assisting in materially strengthening the bone. Towards the 

 extremities of the long bone the structure and arrangement of the bone undergoes 

 a change. There is no marrow cavity, the spongy tissue is not so open and 

 irregular, and the external wall is much thinner than in the shaft ; indeed in many 

 instances it is little thicker than stout paper. A closer examination of the arrange- 

 ment of this spongy tissue throughout the bone suggests a regularity in its 

 arrangement which might escape notice ; and if, in place of one bone only being 

 examined, sections of other bones are also inspected, it will be observed that the 

 spicules of this tissue are so arranged as best to withstand the strains and stresses 

 to which the bone is habitually subjected. 



From what has been said it will be obvious that the arrangements above 

 described are those best adapted to secure the maximum of strength with the 

 minimum of material, and a consequent reduction in the weight of the skeleton. 

 The same description applies, with some modification, to bones of flattened form. 

 Taking as an example the expanded plate-like bones of the cranial vault, their 

 structure, as displayed on section, exhibits the following appearance : The outer 

 and inner surfaces are formed by two compact and dense layers, having sandwiched 

 between them a layer of spongy tissue called the diploe, containing red marrow. 

 Note that there is no medullary cavity, though in certain situations and at certain 

 periods of life the substance of the diploe may become absorbed and converted, by 

 the evagination of the mucous membrane of the respiratory tract, into air-spaces 

 or air-sinuses. 



Structure of Bone (Microscopic). 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. 

 Compact bone merely differs from loose or spongy bone in the denseness of its 

 tissue, the characteristic feature of which is the arrangement of the osseous 

 lamellae 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 

 lamellae are arranged concentrically, separated here and there by interspaces called 

 lacunas, in which the bone corpuscles are lodged. Passing from these lacunae are 

 many fine channels called canaliculi. These are disposed radially to the Haversian 

 canal, and pass through the osseous lamellae. They are occupied by the slender 

 processes of the bone corpuscles. Each Haversian system consists of from three to 

 ten concentric rings of osseous lamellae. 



In addition to the lamellae of the Haversian systems there are others which are 

 termed the interstitial lamellae; these occupy the intervals between adjoining 

 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 lamellae arranged circum- 

 ferentially; these are sometimes referred to as the outer fundamental lamellae. 

 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. 



Ossification of Bone. For an account of the earlier development of the 

 skeleton the reader should consult a manual of embryology. Concerning the 



