244 FORTIFICATION OF CHAMBERS. 



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 no- 

 thing more than air, and must consequently be exposed to 

 great pressure when at the bottom of the sea. Several con- 

 trivances 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 PI. 31, Fig. 

 1, and PI. 32, Fig. 1.,) so as to offer in all directions the 

 greatest resistance to any pressure that tends to force it 

 inwards. 



Secondly, this arch is farther fortified by the addition of 

 numerous minute Ribs, which are beautifully marked in the 

 fossil specimens represented at PI. 32, Fig. 1. In this fossil 

 the external shell exhibits fine wavy lines of growth, which, 

 though individually small and feeble, are collectively of 



ber, and forms a collar, around Uie membranous pipe, thus, directing' its pas- 

 sajre through tlie transverse plates, and also affording to it, wlien dis. 

 tended with fluid, a strong support at each collar. A similar projecting 

 collar is seen in the transverse plate of a fossil Nautilus. (PL32, Fig, 2, 

 e, and Fig. 3, e, i. and PI. 33.) A succession of such supports placed at 

 short intervals from one another, divides this long and thin membranace- 

 ous tube, when distended, into a series of short compartments, or small 

 oval sacs, each sac communicating with tiie 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 PI. 32, Figs. 2, 3, and PL 

 33.) 



The strength of each sac is thus increased by the shortness of the dis- 

 tance between its two extremities, and tiie entire pipe, thus subdivided into 

 thirty or forty distinct compartments, derives from every subdivision an ac- 

 cession of power to sustain the pressure of any fluid that may be introduced 

 to its interior. 



* We learn from Mr. Owen, that there was no possibility of tlie access of 

 water to the air chambers between the exterior of the siphuncle and the 

 siphonic aj)crturcs of the transverse plates; because flic entire circumference 

 of the mantle in which the sipiiuncle oriirinales, is firmly attached to tlie 

 shell by a horny girdle, inipenctrable by any fluid. — Memoir on Nautilus 

 Pompiliiis, p. 47. 



