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. 3.11. 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. 3.11). 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 may 

 appear negative or positive, follows after an interval and is linked with 

 repolarization of the ventricular surface. Contraction of the S.V. is cor- 

 related with a small negative V wave, immediately preceding the P wave. 

 This appears as a simple diphasic wave in the isolated sinus, and stands 

 revealed after extirpation of the ventricle. In selachians a B wave, repre- 

 senting activity of the truncus, is sometimes registered (71, 72, 98). 



The fish heart receives inhibitory fibres in cardiac branches of the vagus 

 nerve, and these terminate around the sino-auricular opening on post- 

 ganglionic neurones which influence the pacemaker. Stimulation of the 

 vagus slows the heart or brings it to a standstill (Fig. 3.12). The vagus is 

 believed to exert its effect on the cardiac musculature, especially the pace- 

 maker of the sinus, by release of a chemical transmitter, acetylcholine. 



