Connective Tissues. 29 



tissue, the periosteum. During the period of growth this mem- 

 brane contains large numbers of bone-forming cells, the osteo- 

 blasts, 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 

 mechanital 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. 10) the central portion 

 or shaft consists of a cylinder of compact 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 cancellous 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 membrane 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 super- 

 ficial 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. 



Cartilage bones, on the other hand, are the characteristic 

 elements of the skeleton. They are known as replacing bones 

 because they are formed on a cartilage basis, the latter, as indicated 

 above, representing the primary cartilage skeleton of the embryo. 

 In the course of development, except in certain regions where the 

 cartilage persists throughout life, the cartilage material is replaced 

 by bone, which tends to surround and also invade it. 



