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COMPOSITION OF BONE. 91 



"the parts, an outer dense layer and an inner spongy por- 

 tion. In the flat and irregular bones there is no medullary 

 cavity, and the whole centre is filled up with cancellated 

 tissue with red marrow in its spaces. For example, in 

 the thin bones roofing in the skull we find an outer 

 and inner hard layer of compact bone known as the outer 

 and inner tables respectively, the inner especially being very 

 dense. Between the two tables lies the spongy bone, 

 red in color to the naked eye from the marrow within it, 

 and called the diploe. The interior of the vertebra? also is 

 entirely occupied by spongy bone. Everywhere, except 

 where a bone joins some other part of the skeleton, it is 

 ^covered with the periosteum 



Chemical Composition of Bone. Apart from the bone 

 corpuscles and the soft contents of the Haversian canals 

 and of the spaces of the cancellated bone, the bony sub- 

 stance proper, as found in the lamellae, is composed of 

 -earthy and organic portions intimately combined, so that 

 the smallest distinguishable portion of bone contains both. 

 'The earthy matters form about two thirds of the total 

 weight of a dried bone, and may be removed by soaking 

 the bone in dilute hydrochloric acid. The organic portion 

 left after this treatment constitutes a flexible mass, retain- 

 ing perfectly the form of the original bone. By long boil- 

 ing, especially under pressure at a higher temperature 

 than that at which water boils when exposed freely to the 

 air, the organic portion of the bone is nearly entirely con- 

 verted into gelatine which dissolves in the hot water. Much 

 of the gelatine of commerce is prepared in this manner by 

 .boiling the bones of slaughtered animals, and even well- 

 picked bones may be used to form a good thick soup if 

 boiled under pressure in a Papin's digester; much nutri- 

 tious matter being, in the common modes of domestic 

 cooking, thrown away in the bones. 



The earthy salts of bone may be obtained free from or- 

 ganic matter by calcining a bone in a clear fire, which burns 

 away the organic matter. The residue forms a white very 

 brittle mass, retaining perfectly the shape and structural 

 details of the original bone. It consists mainly of normal 



