8 STRUCTURE AND FORMATION OF BONE. 
of a looser network of elastic fibres. Beneath this 
second layer is a fibrous network containing, in the 
young bone, numerous protoplasmic nucleated cells 
of a spherical or oblong form ; these are the osteoblasts 
—the active agents concerned in the formation of new 
bone. Those portions of the bones, which enter into 
articulation with others, are covered by hyaline carti- 
lage ; the periosteal layer being absent here. 
The external portion of the bone, the matrix or 
osseous substance, is a dense fibrous connective tissue. 
The matrix—ossein—is largely impregnated with 
various insoluble inorganic calcium salts, chiefly phos- 
phates and carbonates. 
The osseous element is arranged in a series of 
microscopic plates, or lamelle, between which are a 
series of oblong or oval spaces, the lacunze. From 
each of these spaces numerous fine canals—eanaliculi 
—pass out, and fuse with those passing out from the 
lacune above and below. ‘The lacunz and canaliculi 
together form the lymph canalicular system, and com- 
municate with the lymphatic vessels of the marrow. 
In each lacuna there is a flattened protoplasmic 
cell, containing an oval nucleus termed the bone cell. 
In the young bone the cell gives off pseudopodia-like 
branches, which pass into the canaliculi. In the adult 
condition these branches are few ; and the cell is then 
often termed a bone corpuscle. 
In an ordinary shaft of bone (compact bone) the 
