All bones are made up of two parts: 1, an organic matrix; 2, mineral matter or bone-ash. If the rib of a horse be macerated for a few days or weeks in dilute hydrochloric acid, the mineral or earthy matter will be dissolved out of it and the animal or organic matrix will remain behind. In this condition it still retains its original form, but, having lost its hardening constituents, it is now soft and flexible, and may be bent in any direction like a piece of india-rubber, or even tied in a knot.
If a second rib be placed in a bright clear fire and burnt, all the animal matter is destroyed and driven off, leaving the earthy substance behind as a white brittle mass, and, as in the previous experiment, still retaining the shape of the bone.
The relative proportions of organic and inorganic matter entering into the formation of bones vary at different periods of life. In young animals the former makes up nearly one-half of the whole, while in the adult it is reduced to nearly one-third, the remaining two-thirds comprising earthy or mineral substance. It is on account of the larger quantity of soft organic matter they contain that the bones of young animals are so much more yielding, and therefore less liable to break, than those of older ones. The earthy substance of a bone consists of phosphate and carbonate of lime in the proportions of 56 per cent of the former and about 13 of the latter. The animal matrix, which is a kind of gelatine, makes up the rest.