110 



it is very difficult to determine for certain whether a given muscle 

 is really contracting or merely being dragged by its neighbours. 

 This difficulty is especially great in Scyllium, where the muscles are 

 small and very closely pressed together at the base of the fin. It 

 is on this account that both in Naples and in Plymouth I'have based 

 my conclusions on experiments carried out on Raja. In this fish the 

 pectoral fin muscles are larger, more numerous, and less concentrated. 

 Another obvious source of error lies in the possible spread of the elec- 

 tric current from one nerve to another. Braus does not state whether he 

 took special precautions to avoid unipolar excitation, or the leakage 

 of the current. It is on this account that I have always tried to 

 verify my results by repeating the experiments with a mechanical 

 stimulus; a more trustworthy method, since the nerve impulse so 

 generated can not spread from one nerve to another. 



Beaus applied an electrical stimulus at three different points (see 

 his Fig. 3, 1. c, p. 538): 1) near the base of the fin; 2) on the inner 

 side of the body-wall; 3) near the vertebral column. Now, although 

 he states that some 6 or 7 muscles contracted when he stimulated 

 the nerve at the point 1, he acknowledges that when stimulation was 

 applied at the point 3, the results approximated to my own ("Sehr 

 häufig zucken nur zwei benachbarte oder nur ein einziges Muskel- 

 fascikel"). What other explanation can be given of this discrepancy 

 in the results but that the electric current spread from one segment 

 to the next, when applied at the base of the fin? At that point the 

 nerves come very close together and branch. 



Let me now describe my own experiments recently made at Ply- 

 mouth. The skates were killed, the skin rapidly dissected off the 

 muscle of the pectoral fin, and the nerves exposed, either by dissect- 

 ing away the trunk muscles from above, or by opening the abdominal 

 cavity and removing the peritoneum from below. Both methods 

 were pursued with equal success and the same result. About 8 suit- 

 able spinal nerves can thus be cleared and severed from the spinal 

 cord. A large number of experiments were made on numerous skates, 

 confirming some, but not all, of my former conclusions. 



It can easily be made out that the stimulation of one nerve does 

 not produce a general contraction of the fin-muscles, but only a local 

 contraction corresponding in position to the nerve. On stimulating 

 a series of nerves from before backwards the radial muscles are seen 

 to respond in orderly sequence. Moreover, the number of radial 

 muscles in a given region (dorsal or ventral) is exactly double that 



