. The biology of marine animals. Marine animals; Physiology, Comparative. 108 THE BIOLOGY OF MARINE ANIMALS Fish Heart. In the fish heart contractions are initiated in the sinus venosus and spread to other chambers through the myocardium. Other regions capable of producing automatic rhythmic contractions under experimental conditions are: the veins of Cuvier, the auricle, the auriculo- ventricular canal and the ventricle. Local warming or cooling of the pace- makers alters cardiac rate and provides a means of delimiting their boun- daries and observing their influence on other regions. Local w
. The biology of marine animals. Marine animals; Physiology, Comparative. 108 THE BIOLOGY OF MARINE ANIMALS Fish Heart. In the fish heart contractions are initiated in the sinus venosus and spread to other chambers through the myocardium. Other regions capable of producing automatic rhythmic contractions under experimental conditions are: the veins of Cuvier, the auricle, the auriculo- ventricular canal and the ventricle. Local warming or cooling of the pace- makers alters cardiac rate and provides a means of delimiting their boun- daries and observing their influence on other regions. Local warming of the sinus region, for example, increases cardiac rate. When a Stannius ligature, blocking conduction from sinus to ventricle, is applied to the heart of selachians and eels, the anterior chambers beat at a slower rate than the sinus. This effect is not always apparent in the teleost heart. A wave of excitation can proceed in either direction through the myo- cardium. The unidirectional course of excitation, from sinus to auricle to ventricle, results from differences in the rhythmicity of the several regions,. Fig. Electrocardiogram of the Heart of Platichthys flesus PRT waves shown. Calibration, 100 ^V. (From Oets (98).) the sinus showing a more rapid recovery of excitability and thus acting as pacemaker for the whole heart (119). The electrocardiogram (ECG) from the fish heart resembles that of other vertebrates, and gives information about the spread of excitation and contractions of the several chambers. Typically there are a series of slow waves consisting of upward inflexions (negative P, R waves) and downward deflexions (positive Q and S waves) (Fig. ). The electrocardiogram represents a wave front of excitation (depolarization) spreading over the heart. The P wave corresponds to conduction in the auricle, the PQ interval is delay at the auriculo-ventricular junction and the QRS complex represents conduction in the ventricle. A terminal T wave, which
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