STRUCTURE OF BONE. 2 
is as follows: the original cartilage cell divides into 
two, a new capsule being formed around these, the 
old one blending with the matrix ; these two again 
divide, now forming a group of four, each having a 
separate capsule; they may further divide, and form 
a group of eight. The origin of the capsule is but 
imperfectly known; whether or not it is secreted by 
the cartilage cell (Kolliker), or formed from a part of 
the protoplasm of the cell, is not yet settled. 
Bone is a connective tissue, the ground substance 
of which has become impregnated with various calcium 
salts; and which has entered into association with 
other tissues to form a support for the softer parts of 
the body. 
Bone may be either compact, or spongy or cancel- 
fous, the two imperceptibly passing into each other. 
Compact bone is of an ivory nature, and forms the 
peripheral portion of the bone. Spongy or cancellous 
bone forms the articular portions of long shaft-like 
bones. In the bones of the skull—tabular bones— 
the spongy tissue lies between an inner and outer 
layer of compact tissue. In such bones the cancel- 
lated layer is termed azfloé. 
The whole of the skeleton is invested by a fibro- 
vascular membrane, the periosteum. It consists of 
two layers: an outer-fibrous layer, a series of closely 
aggregated bundles of white fibres ramified by blood- 
vessels; and an inner or osteogenetic layer consisting 
