Page:Philosophical Transactions - Volume 096.djvu/14

 When placed near the tail, the single fins seem also to aid the effect of that instrument by increasing its breadth.

The tail is the principal organ of progressive motion, and its actions are performed by the great mass of lateral muscles. There are a series of short muscles for the purpose of changing the figure of the tail fin, which arise from the spine and coccyx, and are attached to the rays immediately beyond their joints: (dd): their action is to expand the rays, and by partial contractions to alter the lateral position of the fin. Slender muscles are placed between the several rays, (ee,) whose office is to converge them previous to the stroke of the tail.

The muscles situated on the head are those, which act on the membrana branchiostega, the under jaw, os hyoides, fauces, and the globe of the eye.

In order to determine the effect of the fins on the motions of fishes, a number of living dace, of an equal size, were put into a large vessel of water. The pectoral fins of one of these fishes were cut off, and it was replaced with the others. Its progressive motion was not at all impeded; but the head inclined downwards, and when it attempted to ascend, the effort was accomplished with difficulty.

The pectoral and abdominal fins were then removed from a second fish. It remained at the bottom of the vessel, and could not be made to ascend. Its progressive motion was not perceptibly more slow; but when the tail acted, the body shewed a tendency to roll, and the single fins were widely expanded, as if to counteract this effect.

From a third fish, the single fins were taken off. This