1. A minimum of 2000 neurones in the medulla of the tench have cyclic activity that is phase-locked to the respiratory cycle.

2. These respiratory neurones are not uniformly distributed throughout the medullary region where they occur. They tend to occur in two bilateral groups, each of which, toward its rostral end, tends to be split into a dorso-lateral and ventro-medial group. Specific patterns of activity are more common in some regions than in others.

3. No evidence was found for anatomically localized groups of neurones with interactions primarily within the group. Neurones within 0.2 mm of each other are more often either synchronous or anti-synchronous than are neurones separated by a distance of 0.5-1.0 mm, but so are neurones separated by 1.0-2.0 mm. Contrary to a speculation by Shelton, respiratory neurones are not bunched into anatomical clusters with a diameter of 0.6 mm or less.

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