40 GENEEAL PART. 



elastic cartilage ; the latter containing a network of elastic fibres. 

 There are also intermediate forms, approximating to the fibrillar 

 connective tissue, in which cartilage cells may be surrounded by 

 bundles of connective tissue fibres. The cells are placed in spaces, 

 which are usually round, in the intercellular substance, and are sur- 

 rounded by firm layers which are separated off from the latter, and 

 have the appearance of capsules. These so-called cartilage capsules 

 were formerly looked upon as the membranes of the cartilage cells, 

 analogous to the cellulose capsules of plant cells ; a view of them 

 which is not in any way opposed by what is known as to their 

 development as secretions of the protoplasm. Nevertheless, the 

 capsules stand in closer relation to the earlier formed intercellular 

 substance which has been produced in the same way, in that they 

 often fuse with it. The growth of the cartilage is accordingly in the 

 main interstitial. We frequently see in the spaces in the cartilage 



several generations of cells 

 surrounded by special capsules 

 placed one within the other. 

 In such cases the secreted cap- 

 sules have remained separate 

 from the intercellular sub- 

 stance. Certain kinds of car- 

 tilage, moreover, have spindle- 

 shaped cells, and sometimes 

 the cells are prolonged into 



FIG. 30. Incrusted cartilage, or cartilage bone. r 



numerous radiating processes. 



Calcareous salts may also be deposited in the intercellular substance 

 in a greater or less quantity. In this way arises the so-called in- 

 crusted cartilage, or the cartilage bone (fig. 30), which in the sharks 

 is present as a persistent form of skeletal tissue, but in the higher 

 vertebrates only as a transitional structure. Cartilage owes its special 

 usefulness as a skeletal tissue to its rigidity. It is sometimes found in 

 the Invertebrata (Cephalopoda, tubicolous worms such as Sabella, 

 Ccelenterata), and very generally in the Yertebrata, whose skeleton 

 always contains a certain amount of cartilage, and in fishes may be 

 exclusively constituted of it (cartilaginous fishes). 



Osseous tissue possesses a still higher degree of rigidity. The 

 intercellular substance is strengthened and hardened by the deposi- 

 tion of carbonate and phosphate of lime, while the cells (the so-called 

 bone corpuscles^ possess numerous fine processes which anastomose 

 with each ether (fig. 31 a, b, c). The cells occupy spaces in the com- 



