TISSUES, ORGANS, AND SYSTEMS. 101 



cells and of a fibrous 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 lacunae and canalicidi 

 by the thickening of their walls, with a contemporaneous development 

 of pore canals, which subsequently grow into the matrix and unite with 

 one another; whilst the matrix of the cartilage and the fibrous sub- 

 stance harden into the matrix of the bone by the deposition of calca- 

 reous 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 periosteum, and, if they be present, by those of the 

 medulla and the Haversian canals also. The bones have a great capa- 

 city of regeneration, and readily unite ; in fact, very great losses of 

 substance 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 osteoidea, or tooth 

 cement. Many cartilages ossify with tolerable regularity as they grow 

 older ; as the costal-cartilages, and those of the larynx. 



Dentine may be regarded as a modification of osseous substance, 

 which, instead of solitary lacunce, 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 sub- 

 stance ; a view which gains additional support from the numerous transi- 

 tional forms, to be observed in animals, between typical dentine and 

 osseous tissue. 



In the Vertebrata, 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 the nose (proboscis of the Pig and Mole, os 

 prcenasale of the Sloth), in the tongue (os entoglossum of Fishes and 

 Birds), in the respiratory organs (laryngeal, tracheal, and bronchial 

 bones of many Birds), in the sexual organs (penis-bone of Mammalia), 

 in the osseous system (ossa sterno-costalia of birds and some mammals). 

 In the Invertebrata true bones are never found, being, in them, replaced 

 by the so-called calcareous skeletons, which principally consist of car- 



