CONNECTIVE TISSUE 



45 



were already present when the first membrane was secreted. Caleifled 

 cartilage is formed by the appearance of calcareous deposits in the 

 intercellular substance. 



Cartilaginous tissue, on account of its firmness, serves as support- 

 ing tissue in vertebrate and in some 

 invertebrate animals. 



Bone tissue forms, jmr excellence, 

 the supporting tissue of vertebrates. 

 The intercellular or bone substance 

 becomes as hard as stone by a com- 

 bination of lime-salts with some ground 

 substance, which yields glue on being 

 boiled, and does not dissolve under 

 treatment with acids. In it are scattered 

 the cell elements (bone cells) ; they are 

 much branched, and connected by their processes ; they are arranged in 

 parallel layers, often concentrically round the cavity (Fig. 44). Bone 

 tissue arises out of indifferent connective tissue cells, which are arranged 

 in strands or flat expanses, and which function as formative cells 

 of the bone tissue, osteoblasts (Fig. 45, a). They produce on one side 

 bone substance, often in the direction of cartilaginous masses, which 

 they supplant, at the same time forming processes which remain im- 



FiG. 43.— Fibrous cartilage, after Glaus. 



Pig. 44.— Bone cells, after Gegenbauer. 



Fio. 46.— Bone tissue, a, Osteoblasts ; 

 !), bone cells (after Gegenbauer). 



bedded in the bone substance. New masses of bone substance being 

 constantly formed from the osteoblasts, some of the latter come to lie 

 in the bone substance, and become bone cells. 



Dentine is nearly related to bone tissue. Here the formative cells 

 (odontoblasts) do not enter into the dentine which they have secreted ; 

 they all remain at its base, but send into it numerous finely branched 

 processes (fibres), which run parallel to each other in as many little 



