AND DEVELOPMENT OF BONE. 195 



the subjects in view, it may not be nmiss to give a ver}'^ brief account 

 of the general structure of bone, in order that every one may be 

 enabled to form an opinion of the nature and importance of the obser- 

 vations r.nd experiments about to be submitted. 



Bone varies in density according to its situation- and the purpose it 

 has to fulfil, being, in some parts of the same bone, light and porous, 

 whilst in other portions it is exceedingly compact and heavy : thus the 

 diaphysis or shaft of a long bone is compact, while the extremities 

 are light and porous. The specimens before you illustrate these ex- 

 treme conditions existing in separate bones ; the heavier is from the 

 head of the Greenland whale, the other is a lumbar vertebra from the 

 adult human subject. The contrast between the two is most striking. 

 Although great diversity exists between bones, in respect to their den- 

 sity, yet there is a wonderful similarity of internal structure throughout 

 the bones of mammalian animals. Dismissing, however, minor pecu- 

 liarities, it will be well, for the purposes of description, to assume as 

 a type of bony tissue, the shaft of the human femur of the adult. 



We find the shaft of the bone to be invested by a membrane which, 

 in some places, is more firmly adherent than in others. This mem- 

 brane consists externally of white fibrous tissue, having a subjacent 

 layer of cells, termed the periosteum. It sends numerous processes 

 into the deep structure of the bone, affording sheaths to the capillary 

 vessels and nerves, so that when torn from the bone, these lacerated 

 processes give to the attached surface of the periosteum an appearance 

 of roughness. The external surface of the periosteum gives attach- 

 ment to the fibrous tendons of muscles which interlace with the fibres 

 of the periosteum. Cutting through the fresh or living bone, at right 

 angles to the direction of the axis of the shaft, we find a large central 

 space called the medullary canal, and which is occupied by a fatty sub- 

 stance, the medulla or marrow. We observe, also, that the cut sur- 

 face of a living bone bleeds from several pores, that is, from the 

 mouths of the vessels contained within their respective canals. If the 

 bone be submitted to the long continued action of fire, all the organic 

 matter of the bone, consisting of fibrous tissue, blood, fat, &c., is 

 burned away, the earthy matter alone remaining ; the bone, however, 

 still preserves its original shape, but has lost about 20 per cent, in 

 weight, so that the earthy matter in the femur of the human adult 

 constitutes about 80 per cent of the total weight of the bone. 



A chemical examination of the inorganic residuum shows it to con- 



