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. 



