STKUCTUEE OF BONE. 83 



greater crushing strain ; it is stated that a cubic 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 have been excavated in Egypt. 



Structure of Bone (Macroscopic). To obtain an idea of the structure of a 

 bone it is necessary to examine it both in the fresh or 'recent condition and in the 

 macerated state. In the former the bone is covered by a membrane which is with 

 difficulty torn off, owing to the abundance of fine fibrils which enter the substance 

 of the bone from its deep surface. This membrane, called the periosteum, overlies 

 the bone, except where the bone is coated with cartilage. This cartilage may form 

 a bond of union between contiguous bones or, in the case of bones united to each 

 other by movable joints, may be moulded into smooth articular surfaces called the 

 articular cartilages. The attachment of the various ligaments and muscles can also 

 be studied, and it will be noticed that where tendon or ligament is attached, the 

 bone is often roughened to form a ridge or eminence ; where fleshy muscular fibres 

 are attached, the bone is, as a rule, smooth. In the macerated condition, when 

 the cartilage and fibrous elements have been destroyed, it is possible, however, 

 to determine with considerable accuracy the parts of the bone covered with 

 articular cartilage, since the bone here is smooth and conforms generally to the 

 curves of the articular areas of the joint ; these areas are referred to as the 

 articular surfaces of the bone. The bone, stripped of its periosteal covering, 

 displays a dense surface finely pitted for the entrance of the processes derived from 

 the periosteum, which thus establish a connexion between the bony substance and 

 that vascular layer ; here and there, more particularly in the neighbourhood of the 

 articular extremities, these pits increase in size and number and allow of the trans- 

 mission of small blood-vessels. If careful examination is made, one or two foramina 

 of larger size will usually be noticed. These vascular foramina or canals allow the 

 passage of arteries of considerable size into the interior of the bone, and are called 

 the canales nutricii or nutrient canals or foramina of the bone. There are also 

 corresponding channels for the escape of veins from the interior. 



In order more fully to ascertain the structure of bone it will be necessary to 

 study it in section. Taking first a long bone, such as one meets with in the 

 limbs, one notices on longitudinal section, that the bone is not of the same 

 density throughout, for, whilst the external layers are solid and compact, the 

 interior is made up of loose spongy bone called sulstantia spongiosa (cancellous 

 tissue). Further, it will be observed that in certain situations this spongy substance 

 is absent, so that there is a hollow in the interior of the bone called the medullary 

 cavity. In the recent condition this cavity is filled with the marrow and is hence 

 often called the marrow cavity. This marrow, which fills not only the marrow cavity 

 but also the interstices between the fibres of the spongy substance, consists largely of 

 fat cells, together with some marrow cells proper, supported by a kind of retiform tissue. 

 The appearance and constituents of the marrow differ in different situations. In the 

 medullary cavity of long bones the marrow, as above described, is known as medulla 

 ossium flava (yellow marrow). In other situations, viz., in the diploe of the cranial 

 bones (to be hereafter described), in the spongy tissue of such bones as the vertebrae, 

 the sternum, and the ribs, the marrow is more fluid, less fatty, and is characterised 

 by the presence of marrow-cells proper, which resemble in some respects colourless 

 blood corpuscles. In addition to these, however, there are small reddish-coloured 

 cells, akin to the nucleated red corpuscles of the blood of the embryo. These cells 

 (erythroblasts) are concerned in the formation of the coloured corpuscles of the 

 blood. Marrow which displays these characteristic appearances is distinguished 

 from the yellow variety, already described, by being called the medulla ossium rubra 

 (red marrow). The marrow met with in the spongy tissue of the cranial bones of 

 aged individuals often undergoes degenerative changes and is sometimes referred to 

 as gelatinous marrow. 



