10 THE VERTEBRATE SKELETON. 



The commonest kind of cartilage, and that which pre- 

 forms so many of the bones of the embryo, is hyaline cartilage. 

 It consists of oval nucleated cells occupying cavities (lacunae) 

 in a clear intercellular semitransparent matrix, which is pro- 

 bably secreted by the cells. Sometimes one cell is seen in each 

 lacuna, sometimes shortly after cell-division a lacuna may con- 

 tain two or more cells. The free surface of the cartilage 

 is invested by a fibrous membrane, the perichondrium. 



Bone consists of a series of lamellae of ossified substance 

 between which are oval spaces, the lacunae, giving rise to 

 numerous fine channels, the canaliculi, which radiate off in 

 all directions. The lacunae are occupied by the bone cells 

 which correspond to cartilage cells, from which if the bone is 

 young, processes pass off into the canaliculi. It is obvious 

 that the ossified substance of bone is intercellular in character, 

 and corresponds to the matrix of cartilage. 



Bone may be compact, or loose and spongy in character, 

 when it is known as cancellous bone. In compact bone 

 many of the lamellae are arranged concentrically round 

 cavities, the Haversian canals, which in life are occupied by 

 blood-vessels. Each Haversian canal with its lamellae forms 

 a Haversian system. In spongy bone instead of Haversian 

 canals there occur large irregular spaces filled with marrow, 

 which consists chiefly of blood-vessels and fatty tissue. The 

 centre of a long bone is generally occupied by one large con- 

 tinuous marrow cavity. The whole bone is surrounded by 

 a fibrous connective tissue membrane, the periosteum. 



THE DEVELOPMENT OF BONE. 



Periosteal ossification. An example of a bone en- 

 tirely formed in this way is afforded by the parietal. The 

 first trace of ossification is shown by the appearance, below 

 the membrane which occupies the place of the bone in the 

 early embryo, of calcareous spicules of bony matter, which 

 are laid down round themselves by certain large cells, the 



