In the arrangement of the fibres in different bones, so as to adapt them to the specific offices they have to servo, there is exquisite mechanism. 1Vhere the principal object is either extensive protection, or the provision of broad surfaces for the attachment of muscles, the osseous fibres are so disposed as to form flattened plates, as in the bones of the skull. 1Vhen on the other hand a system of levers is wanted, as in the limbs which have to sustain the weight of the trunk, and to confer extensive powers of locomotion, the bones are modelled into lengthened cylinders, generally somewhat expanded at the extremities for greater convenience of mutual connection. The shank or body of this hollow cylinder consists principally of compact with but little spongy matter, while the extremity or head of it is principally composed of spongy matter, with only a thin cruet of compact substance. The principal mechanical property required in every cylindrical lever is rigidity, and more especially tho power of resisting forces applied transversely, that is, tending to break the cylinder across ; it has been often stated that a given quantity of materials could not possibly have been disposed in a manner better calculated for such resistance than them) in the form of a tube or hollow cylinder. The hollow stems of vegetables derive their chief strength from possessing this form. Bones also are rendered both lighter and stronger by being made hollow than if the cylinder had been solid ; and as it in in the middle of the shaft that the strain is greatest, so it is hero that the cavity is largest and the resistance most effectual.
Bone has been recently submitted to rigid chemical analysis, and the result is that in healthy bone about two-thirds, or 661 per cent., consist of saline or earthy matters, and 33.3 per cent., or ono-third, of animal matter. The bones of children contain more animal matter, and of aged persons more earth. Different bones also in the same skeleton contain different proportions of constituents. Thus, according to Dr. G. 0. Reel, the bones of the head and the limbs contain more earth than those of the trunk. The following are two ultimate analyses of bone from Dr. Sharpey's Introduction to 'Quain's Anatomy the one is by Berzelius, the other by Mr. Middleton of University College :— Berzeli us. NI iddleton Animal Matter 33.30 Phosphate of Limo . . . . 61'11 Carbonate of Lime 11.30 Fluoride of Calcium . . . . 2.00 • Magnesia, wholly or partially in the state } of a phosphate Soda and Chloride of Sodium . . . P20 P68 The phosphate of lime in bones is peculiar, and is known amongst chemists as the ' bone-earth phosphate.' It is what is called a tribasie phosphate, consisting of 8 equivalents of lime, 3 of phosphoric acid, and 10 of water. Fluoride of calcium is found in larger quantities in fossil than in recent and such is its abundance in sonic fossils, as those brought from the Sevalik Hills, in India, by Dr. Falconer, that we must have recourse to the supposition of a
substitution of this substance for phosphate of lime, in order to account for itspresence.
We have referred to the statement that the bones of the limbs contain snore earth than the trunk. The following analysis by Lehmann shows that the bones of the arms and legs have a different compostion Humerus. Femur.
Phosphate of Lime and Fluoride of Calcium Carbonate of Lime . . Phosphate of Magnesia . . . , . 1.08 1.00 Chloride of Sodium . . . . . 0'40 Soda . . . 1.35 1'04 Organic Matter . . . . . . 31'52 28'61 When examined under the microscope, sections of the different bones present very different appearances, not at first easily reconciled with a common mode of origin or growth. The result of a clone investigation, however, shows that the osseous tissue like all other parts of the body originates in cells. This fact is not easily traced, and we shall first speak here of the appearances presented on a minute examination of the texture of the bone.
The canals which are everywhere found traversing variously the substance of bone, and giving pap age to the blood-veesels for the nourishment of the tissue, are called Haversian Canals, a name given them in consequence of Clopton Havers having been the first who gave a full account of them. The parietes of these canals have a laminated arrangement. The Inminte themselves are numerous and placed concentrically ; the internal lamina, that which is in imme diate contact with the wand or vessels, being the most distinctly marked, and each succeeding one having a loss distinct outline.
Besides the concentric lamina', there are others which surround the exterior of the bone, and may he known as the superficial lamina'. In connection with both the concentric and superficial Iamb= are a third set, which cannot belong to either of the other orders, but which are placed between them, and form the bond of union between each system.
Much has been lately written on the bone-corpuscles. These are small cells of oval form placed between the laminar, and haring numerous distinct tubes running from them in almost every direction. They have been sometimes compared to a spider with many legs. The corpuscles, or as they are occasionally called the calcigerous cells, have a definite relation to the Haversian Canals and to each other.
The Haversian Canals, the Osseous ',aniline, and the Bone-Corpuscles are therefore the leading points to be mentioned in treating of the structure of the bone. Upon a closer view, however, it will be seen that it is only the Imbue which are bone ; the canals and corpuscles aro spaces existing in bone, and are not really necessary to the existence of osseous tissue, though they are requisite where the amount of substance is appreciable to the unaided senses.