24 INTRODUCTION. 
which is especially noticeable as one of the places of formation of red 
blood-corpuscles. Externally every bone is covered by a layer of 
fibrous connective tissue, the periosteum. 
The dentine of teeth and placoid scales is closely allied to bone, 
the chief difference in density, the bone-forming cells (odontoblasts) 
not being enclosed in the matrix, while the canaliculi (here called den- 
tinal canals) are parallel to each other. 
Blood is sometimes regarded as a connective tissue, the corpuscles 
being the cells and the fluid part (plasma) the matrix. It is here dealt 
with in connection with the circulatory system. 
