Skeletal, Structures. 



15 



The bone' materials are deposited in layers or lamellae, which are com- 

 parable to highly modified white fibres of fibrous connective tissue. The 

 lamellae enclose between them greatly branched spaces, the lacunae, in 

 which in the natural condition the bone-cells are accommodated. On 

 account of certain differences in development, the lamellae are disposed in 

 different ways. Some are disposed concentrically around longitudinal 

 spaces, the Haversian canals, forming in this way the so-called Haversian 

 systems. The canals are occupied in the natural condition by bloodvessels. 

 Others of them, the interstitial lamellae, are disposed in a somewhat 

 parallel fashion between the Haversian systems ; while a third series, the 

 circumferential lamellae, are disposed in a parallel fashion with respect to 

 the periphery or the internal surface of the bone. 

 In the natural condition the bone is enclosed 

 except on its articular surfaces by a layer of con- 

 nective tissue, the periosteum. During the period 

 of growth this membrane contains large numbers 

 of bone-forming cells, the osteoblasts, through the 

 activity of which the deposition of the bone lamellae 

 is accomplished. 



It is only in a few cases that the bones of the 

 skeleton may be looked upon as solid structures — 

 as a rule they consist of a fairly thin shell of hard 

 or compact bone surrounding a central mass of 

 spongy or cancellous bone. This arrangement is 

 one of great mechanical strength, combined with 

 lightness, and at the same time provision is made 

 in the interior of the bone for bloodvessels and 

 marrow spaces. Thus in a long bone (Fig. 9) the 

 central portion or shaft consists of a cylinder of com- 

 pact bone surrounding an extensive space, the 

 marrow-cavity. The ends or extremities consist 

 each of a thin shell of compact bone continuing 

 that of the shaft and surrounding a mass of can- 

 cellous tissue. In the short, flat, or irregular bones 

 of the skeleton no continuous marrow cavity is 

 formed. 



In point of origin bones are of two chief kinds 

 — membrane or derm bones, and cartilage or 

 replacing bones, but a third group is represented 

 by the tendon or sesamoid bones which are 

 developed in the tendons of muscles. The mem- 

 brane bones are few in number. They comprise the roofing and 

 facial bones of the skull, most of which are distinguished by their 

 flattened, tabulate character. They are formed in connective 

 tissue membranes, and although they sometimes contain cartilage they 

 are not formed on a cartilage basis. Moreover, they are elements lying 

 in a superficial position with respect to the skeleton proper. They owe 

 this condition, and also their designation as derm bones, to the circum- 

 stance that they represent surface plates which in lower vertebrates are 

 associated with the skin. 



"\ 



Fig. 9. Divided femur 

 of the rabbit: at., can- 

 cellous tissue; e., distal 

 epiphysis; e.l., epiphysial 

 line; m.c., marrow-cavity; 

 s.. shaft. 



