4 Mr. Carlisle's Lecture on the Arrangement 
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 chang- 
ing 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 par- 
tial 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 
* Cyprinus leuciscus. 
