TISSUES, ORGANS, AM) SYSTEMS. 85 



substance similar to connective tissue. In both cases it is the 

 cells — in the one the cartilage cells, in the other, cells without 

 any defined character — which form the lactone and canalicu/'t by 

 the thickening of their walls, with a contemporaneous develop- 

 ment of pore canals, which subsequently grow into the matrix 

 and unite with one another; whilst the matrix of the cartilage 

 and the fibrous substance harden into the matrix of the bone 

 by the deposition of calcareous salts, which likewise infiltrate 

 the thickened cell-walls. The nutrition of the bones is very 

 energetic, and is effected by the vessels of the investing peri- 

 osteum, and, if they be present, by those of the medulla and the 

 Haversian canals also. The bones have a great capacity of re- 

 generation, and readily unite j in fact, very great losses of sub- 

 stance are repaired, or even whole bones, if the periosteum be 

 left : adventitious development of bone is also very common. 



The osseous tissue is found, firstly, in the bones of the 

 skeleton, to which also the auditory ossicles and the hyoid bone 

 belong ; secondly, in the bones of the muscular system, as the 

 sesamoid bones and the ossifications of tendons ; thirdly, in the 

 substantia osteoidta, or tooth cement. Many cartilages ossify 

 with tolerable regularity as they grow older ; as the costal-carti- 

 lages, and those of the larynx. 



Dentine may be regarded as a modification of osseous sub- 

 stance, which, instead of solitary lacuna, presents long canals, 

 — the dental canals ; besides which, it exhibits some chemical 

 modifications. The development of the dentine leads to the 

 conclusion that it is an osseous structure, whose cells, in the 

 course of their ossification and thickening, become united 

 into tubes, and have very little or no intermediate substance ; 

 a view which gains additional support from the numerous tran- 

 sitional forms, to be observed in animals, between typical den- 

 tine and osseous tissue. 



[In the Yertebrata, bone is found more extensively distributed 

 than in man. It exists in the skin (Armadillo, Tortoises, Lizards, 

 Fishes), in the heart (the cardiac bone of the Ruminants and 

 Pachydermata), in the muscular system (diaphragmatic bone of 

 the Camel, Lama, and Porcupine, ossified tendons of birds), in 

 the eye (sclerotic ring of Birds, Chelonians and Saurians, bony 

 scales of the sclerotic of many Fishes), in the external portion of 



