Abstract
ABSTRACT The hearts of both elasmobranch and teleost fish are known at times to beat at a particular phase of the respiratory cycle and thus to exhibit a regular rhythm locked to that of respiration (Lyon, 1926; Lutz, 1930; Satchell, 1960; Shelton & Randall, 1962). In the laboratory, the hearts of Squalus acanthias and Mustelus antarcticus usually exhibit a 1:2,1:3 or 1:4 rhythm, the P wave of the electrocardiogram occurring as the mouth opens in the appropriate respiratory cycle. Lyon (1926) reported a 1:1 co-ordination in Carcharias sp. In Squalus acanthias it is known that respiration stimulates receptors in the pharynx, the afferent fibres of which pass up the branchial branches of the glossopharyngeal and vagus nerves to the medulla; the efferent limb of the reflex is the cardiac vagus nerve (Satchell, 1960).
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