File:Animal and vegetable physiology, considered with reference to natural theology (1836) (14578297597).jpg

Identifier: animalvegetable01roge (find matches)
Title: Animal and vegetable physiology, considered with reference to natural theology
Year: 1836 (1830s)
Authors: Roget, Peter Mark, 1779-1869
Subjects: Biology Physiology Plant physiology Natural theology
Publisher: Philadelphia, Carey, Lea & Blanchard
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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rs usually resortedto for effecting the same object. We have seen that if thebody of a fish were heavier than an equal bulk of water, andif no muscular exertions were made, it must necessarily de-scend in that fluid. If, on the contrary, it were specificallylighter, it would as necessarily rise to the surface. Werethe animal to acquire the power of altering, at pleasure, itsspecific gravity, it would then possess the means of risingVol. I. 38 298 THE MECHANICAL FUNCTIONS. or sinking, without calling into action either the fins or thetail. Such is precisely the object of a peculiar mechanism,which nature has provided in the interior of the body of thefish. A large bladder, filled with air, has been placed im-mediately under the spine, in the middle of the back, andabove the centre of gravity. This is known by the nameof the air-bladder, or the swhnming bladder, and in thecod-fish it is called the sound. It frequently, as in theCarp, consists of two bladders (a, b. Fig. 195) joined end-
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wise, and communicating with each other by a narrow neck.*When distended with air, it renders the whole fish specifi-cally lighter than the surrounding water; and the fish isthus buoyed up, and remains at the surface without any ef-fort of its own. On compressing the bladder, by the actionof the surrounding muscles, the included air is condensed,the specific gravity of the whole body is increased, and thefish sinks to the bottom. On relaxing the same muscles,the air recovers its former dimensions, and the fish is againrendered buoyant. Can there be stronger evidence of de-sign than the placing of this hydrostatic apparatus, actingupon philosophical principles, in the interior of the organi-zation, for a purpose so definite and unequivocal? In several tribes of fishes there is a canal (c d) establish-ing a communication between this bladder and the stomach,or the gullet (o;) so that by compressing the bladder, a quan-tity of air may be forced out, and a very sudden increase of * There

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