28 



INTRODUCTION 



bone are the cells, arranged in layers. The space occupied by a cell 

 is called a lactina, from which minute tubules or canaliculi penetrate 

 the matrix. There are small spaces in many bones occupied by the 

 red marrow, which is especially noticeable as one of the places of for- 



FiG. i8. — Hyaline cartilage. 



mation 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) 



Fig. 19. — A, Stereogram of bone; B, cross-section of bone, more enlarged; c, canaliculi; 

 bl, bone lamellae; h, Haversian canal; I, lacuna. 



not being enclosed in the matrix, while the canaliculi (here called 

 dentinal 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 connexion with the circulatory system. 



