136 THE APPEARANCE OF JAWS v. 2- 



the effect of the fish on the water and hence on the turbulence in the 

 flow of water and the resistance that must be overcome. Gray has 

 shown that in a dolphin the resistance cannot be that of a rigid model 

 towed at the speed at which the animal moves, since this would require 

 that the muscles generate energy at a rate at least seven times greater 

 than is known in the muscles of other mammals. By watching the 

 flow of particles past the body of fish-like models he showed that 

 movements such as those produced in swimming accelerate the water 

 in the direction of the posterior end, and this would greatly reduce 

 the turbulence. 



Something is known of the nervous mechanism responsible for the 

 production of the swimming waves. An eel can swim if its whole skin 

 has been removed. If a region of the body is immobilized by a clamp, 

 swimming waves can pass along. Therefore the rhythm is determined 

 by some intrinsic activity of the spinal cord and not by any mechanism 

 such as proprioceptor impulses arising in active muscles and causing 

 others to contract. 



Experiments in which the spinal cord was cut across show that in 

 the eel the rhythm is only initiated when suitable impulses reach the 

 cord either from spinal afferents or from the brain. Thus the spinal 

 eel can be made to swim either by fixing a clip on to its caudal fin or by 

 electrical stimulation of the cut end of the spinal cord. Though the 

 cord requires such afferent stimuli for its functioning, they do not 

 determine the frequency of the rhythm, which bears no relationship 

 to that of the applied stimuli. 



In the dogfish the isolated spinal cord is able to initiate rhythmic 

 swimming. After transection behind the brain the posterior portion 

 of the fish exhibits continuous swimming movements for many days. 

 Light touch on the sides of the body inhibits these movements, but some 

 sensory impulses are necessary for their initiation; after complete de- 

 afferentation, by section of all the dorsal roots, the movements cease. 



The information available does not yet enable us to understand 

 fully how the swimming rhythm is initiated and maintained, nor how 

 it is influenced by the brain. It would be very interesting to have 

 further knowledge on these topics, especially because the locomotor 

 rhythms of land animals are probably based on the serial contractions 

 of their fish ancestors. 



3. Equilibrium of fishes in water ; the functions of the fins 



Making use of the methods of investigation of aeronautical engineers, 

 studies have been made of the forces that operate to keep a fish stable 



