i8o 



ANIMALS 



in such large quantities that the cells themselves come to lie 

 far apart in the jelly. Cartilage is sometimes semi-transparent. 

 Sometimes it contains fibres, and very often it is hardened 

 by deposits of lime salts. 



426. When true bone is formed, it either takes the place of 

 cartilage or else is formed where no cartilage had previously 

 existed. In the first case, the cartilage is first dissolved and 

 in its place solid masses of Ume salts are laid down, layer upon 



layer, by special bone-forming 

 cells. Some cells become em- 

 bedded in the bone and these 

 are connected with each other 



by slender protoplasmic threads. 

 The cells and their connecting 

 threads form the lacunae and 

 canaliculae of the dry bone. 

 Around the blood vessels the 



Fig. 94.— Cartilage cells lying singly, bone is deposited in concentric 

 or in small groups of two or three in layers, but elsewhere the layers 



the cartilage jelly which is secreted by -^ ■^ 



them. are parallel with the surface of 



the bone. The layers are called 

 lamellae; the spaces occupied by the blood vessels, Haversian 

 canals. 



427. When the bone does not take the place of a cartilage, 

 it is formed in connective tissue. Such bone is called mem- 

 brane bone. 



428. The skeleton is bound together by bands of exceedingly 

 strong, elastic connective tissue called ligaments. They are 

 found at the joints, binding bone to bone, so as to keep each in 

 its place. They are not connected with the muscles. 



429. The muscles are sometimes connected directly with the 

 bones, sometimes indirectly through the medium of tendons, 

 which are bands of inelastic connective tissue. The muscles 

 which produce motion at a given joint must be connected 



