BODY FLUIDS AND CIRCULATION 



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nerves have also been demonstrated in various amphineurans, gastropods 

 and lamellibranchs : these arise from the pleuro-visceral cords in chitons, 

 and the visceral ganglia of prosobranchs and tectibranchs. Stimulation of 

 these nerves causes acceleration of the heart, or initiates contractions in a 

 quiescent organ. 



In cephalopods both branchial and systemic hearts receive inhibitory 

 fibres. Stimulation of the visceral nerves causes slowing or inhibition of 

 the heart. The distribution of the two visceral nerves is such that each 

 nerve causes inhibition of ventricle and ipsilateral auricle. Reflex afferent 



Fig. 3.15. Inhibition of the Heart of Mercenaria mercenaria by 

 Stimulation of the Visceral Ganglion 



(a). Brief bursts of shocks; figures indicate position of secondary of induction coil 

 (in centimetres), (b). Effect of repeated bursts of shocks. Time scale in (a) 3 sec. (From 

 Prosser (104).) 



pathways linking the two auricles are believed to traverse the visceral 

 nerves. Evidence is also available that the cephalopod heart is provided 

 with acceleratory fibres. In some preparations stimulation of the visceral 

 nerves produces acceleration of rhythm in the systemic and branchial 

 hearts, and augmentation of beat in the systemic ventricle (Eledone, 

 Octopus). Under repetitive stimulation the amplitude of beat rises with 

 increase in number of stimuli, the augmentation amounting to 40-300% 

 with thirty stimuli (Octopus). It is believed that both inhibitory and accel- 

 eratory fibres coexist in the cardiac nerves, the latter showing higher 

 threshold to electrical stimulation, and that these two categories of fibres 

 exert their specific effects by different chemical transmitters (26, 43, 44, 45, 

 46,77, 105, 121, 126). 



