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 
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. 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. 
