i110 PHILOSOPHY OF ZOOLOGY. 
to age, the carthy matter beimg smaliest in youth, but, like- 
wise, according to the nature of the bone itself, and the 
purposes which it is destined to serve. The teeth contain 
the largest portion of earthy matter. Remarkable diffe- 
rences are hkewise observable, according to the class or spe- 
cies. The bones of quadrupeds and birds contain a much 
greater proportion of earthy matter than those of reptiles or 
fishes. In some fishes, the earthy matter is so small, that 
the cartilage continues, during the whole life of the ani- 
mal, soft, flexible, and elastic, as the spme of the lamprey, 
or a little more indurated, as in the bones of the skate and 
shark. These fishes have been termed cartilaginous. 
Even in those fishes which are termed osseous, the cartilage 
bears a much greater proportion to the earthy matter than 
in quadrupeds. 
Bone is readily reproduced, in small quantities, especial- . 
ly in youth. In the case of fracture, the periosteum in- 
flames and swells, the crevice is filled up by a cartilaginous 
basis, abounding in vessels, and the earthy matter is at 
length deposited, giving to the fractured part, in many 
cases, a greater degree of strength than it origmally pos- 
sessed, 
In animals of the deer kind, the horns, which are true 
bone, are annually cast off; a natural jomt forming at their 
base, between them and the bones of the cranium, with 
which they are connected. They are afterwards reproduced 
under a skin or periosteum, which the animal rubs off when 
the new horns have attained their proper size. 
Tn some cases of disease, the earthy matter is again ab- 
sorbed into the system, the cartilaginous basis predomi- 
nates, and the bones become soft and tender. This takes 
place in the disease of youth, termed Rickets, and in a si- 
“milar complaint of advanced life, known under the name of 
Mollities ossium. Yn other mstances, bone 1s formed as a 
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