Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/248

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As neither the siphuncle, nor the external shell have any kind of aperture through which a fluid could pass into the close chambers, it follows that these chambers contain nothing more than air, and must consequently be exposed to great pressure when at the bottom of the sea. Several contrivances are therefore introduced to fortify them against this pressure.

First, the circumference of the external shell is constructed every way upon the principles of an Arch, (see Pl. 31, Fig. 1, and Pl. 32, Fig 1.) so as to offer the greatest resistance to pressure tending to force it inwards in all directions.

Secondly, this arch is further fortified by the addition of numerous minute Ribs, which are beautifully marked in the fossil specimens represented at Pl. 32, Fig 1. In this fossil the external shell exhibits line wavy lines of growth, which, though individually small and feeble, are collectively of

sage through the tans verse plates, and also affording to it, when distended with fluid, a strong support at each collar. A similar projecting collar is seen in the transverse plate of a fossil Nautilus. (Pl. 32, Fig. 2, e, and Fig. 3, e, i. and Pl. 33.) A succession of such supports placed at short intervals from one another, divides this long and thin membranaceous tube, when distended, into a series of short compartments, or small oval sacs, each sac communicating with the adjacent sacs by a contracted aperture or neck at both its ends, and being firmly supported around this neck by the collar of each transverse plate. (See Pl. 32, Figs, 2, 3, and Pl. 33.)

The strength of each sac is thus increased by the shortness of the distance between its two extremities, and the entire pipe, thus subdivided into thirty or forty distinct compartments, derives from every subdivision an accession of power to sustain the pressure of any fluid that may be introduced to its interior.