Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . espine, the pelvis, and more especially in thevaulted roof of the skull, and in the carapace,or upper shell, of the tortoise. The construction of these levers evinces thata minute attention has been bestowed on everycondition by w hich mechanical advantage couldbe gained. In the more perfect developementsof structures, such as those which obtain in thehigher orders of mammalia, and also in the classof birds, all the long bones arehollow cylinders; and their ca-vity is largest in the middle
Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . espine, the pelvis, and more especially in thevaulted roof of the skull, and in the carapace,or upper shell, of the tortoise. The construction of these levers evinces thata minute attention has been bestowed on everycondition by w hich mechanical advantage couldbe gained. In the more perfect developementsof structures, such as those which obtain in thehigher orders of mammalia, and also in the classof birds, all the long bones arehollow cylinders; and their ca-vity is largest in the middle oftheir length. This is shown inFig. 172, which represents alongitudinal section of a humanthigh bone, and in Fig. 173,which is a similar section of thehumerus, or bone of the walls of these bones consistof a dense and compact sub-stance, formed by the close co-hesion of the osseous walls are of greater thick-ness in the middle of the shankor shaft of the column, and be-come thinner as we follow themtowards either of the ends. Thisgradual diminution in the thickness of the walls. 374 THE MECHANICAL FUNCTIONS. arises from the continual separation of theplates, which bend inwards, and crossing eachother, leave a multitude of irregular spaces orcells, which are called cancelli. The plates, pro-ceeding from each side obliquely inwards, atlength meet each other in the axis of thecylinder, so as to close the middle cavity nearthe extremities of the bone, where this spongyor cancellated structure is found to occupy itswhole diameter. Now if we consider that the principal me-chanical property required in every cylindricallever is rigidity, and more especially the powerof resisting forces applied transversely, that is,tending to break the cylinder across, we shallsoon perceive, that a given quantity of ma-terials could not possibly have been disposed ina manner better calculated for such resistancethan when in the form of a tube, or hollow cy-linder.* To this mechan
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