. Animal and vegetable physiology considered with reference to natural theology. Natural theology; Physiology; Plant physiology; Biology. 210 THI2 MECHANICAL FUNCTIONS. direction of the spire : while, on the contrary, those J07 of the intermediate plate form con- centric rings round the cone pa- rallel to its base. Thus the fibres of each layer are at right angles to those of the layer which is con- tiguous to it; an arrangement admirably calculated for giving strength to the shell, by opposing a considerable cohesive resistance to all forces tending to break it, in whatever direc- tion they m
. Animal and vegetable physiology considered with reference to natural theology. Natural theology; Physiology; Plant physiology; Biology. 210 THI2 MECHANICAL FUNCTIONS. direction of the spire : while, on the contrary, those J07 of the intermediate plate form con- centric rings round the cone pa- rallel to its base. Thus the fibres of each layer are at right angles to those of the layer which is con- tiguous to it; an arrangement admirably calculated for giving strength to the shell, by opposing a considerable cohesive resistance to all forces tending to break it, in whatever direc- tion they may be applied.* We here find that a prin- ciple, which has only of late years been recognised and applied to the building of ships, namely, that of the diagonal arrangement of the framework, and the oblique position of the timbers, is identical with that which, from the beginning of creation, has been acted upon by nature in the construction of shells. When the form of the crystals is prismatic, the fibres are short, their direction is perpendicular to the surface, and the prisms are generally hexagonal. This structure is observable in the Teredo gigautea from Sumatra,! and also in many bivalves, such as those belonging to the genera Avicula and Pinna. * The lines indicating the direction of the fibres are shown in the diagram, Fig. 107, which represents a longitudinal section of a shell of this kind. A is the outer layer, of which the fibres pass obliquely downwards. B is the middle layer, having fibres placed at right angles with the former. C is the third, or inner layer, the fibres of which have a direction similar to the outer layer. Within this layer, there is frequently found a deposit of a hard, transparent, and appa- rently homogeneous calcareous material, D. Of this latter substance I shall afterwards have occasion to speak. t In this shell the crystalline appearance is so perfect, that when some fragments were sent to England, they were mistaken for a mineral product
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