Reflex and Rhythmical Movements in the Dogfish

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Abstract
1. The active part played by the two dorsal and anal median fins, and the passive part played by the caudal fin in the normal swimming of Scylliorhinus are emphasised. 2. It is pointed out that when the nerve cord of Scylliorhinus is transected at any level, that part of the trunk below the level of transection produces rhythmical swimming movements. This is interesting when compared with certain teleosts, in which rhythmical swimming movements are only produced when the transection of the cord is at the anterior end of the medulla. In these teleosts the neurons controlling swimming are probably confined to the medulla, while in the dogfish they are dispersed throughout the length of the cord, representing a more primitive condition. 3. The movements are accentuated during and after mechanical stimulation, the tip of the caudal fin and the ventral region near the cloaca being the most sensitive regions. 4. Mechanical stimulation of certain regions produces well-defined reflexes of the body which are superimposed upon the rhythmical swimming movements. These distortions of the normal swimming movements thus produced are interpreted as foreshadowing the more marked, typical reflexes seen in the teleosts. 5. The swimming movements may be inhibited by holding either of the dorsal fins. On release of the inhibitory stimulus, a post-inhibitory rebound in the swimming movements of the fish occurs. 6. A regular secondary periodic rhythm superimposed upon the normal swimming rhythm of the fish is described. 7. On supplying the dogfish with the nerve cord transected at the anterior end of the medulla with extra carbon dioxide, or stopping the oxygen supply, the Cheyne-Stokes rhythm, which is typical of respiration in mammals under abnormal conditions of oxygen and carbon dioxide supply, was found to take the place of the normal swimming rhythm. In spinal fish no such change was recorded. Consequently it is concluded that the neurons in the medulla which control the respiratory movements in some way also influence those responsible for the trunk musculature, so that this typical respiratory rhythm was translated into the swimming rhythm.

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