558 Cotmparative Animal Physiology 



The distribution of inhibitory and accelerator nerves differs among various 

 molluscs. In the pelecypods Mya, Anodonta;^^ and Venus,-' stimulation of 

 the visceral gangUon results in cardiac inhibition. The palliovisceral con- 

 nectives are accelerators, and no inhibitory fibers have been found in chitons, 

 Aplysia, the prosobranchs Haliotis and Natica, the tectobranchs Bulla and 

 Pleurohranchea, the nudibranch Archidoris, or in Ariolimax:^^ The heart 

 in Helix, Umax, and the nudibranch Triopha receives both inhibitory and 

 augmentor nerves from the pleural ganglion. In Aplysia, stimulation of the 

 pleurovisceral nerves accelerates and raises the tone of the heart, an effect 

 which is suppressed by caffeine."^" 



In cephalopods, inhibition of both the systemic and branchial hearts by 

 the visceral nerves was first seen by Paul Bert in 1867. In Octopus,'^^ stimu- 

 lation of a visceral nerve results in slowing of the heart; the two nerves are 

 homolateral with respect to the auricles, but overlap in ventricular action. 

 Synchrony between the two branchial hearts and between the auricles is 

 nervous.*'"'^' '^^ A single inhibitory nerve volley may delay one heart beat, an 

 effect prevented by curare. The visceral fibers terminate in a cardiac gan- 

 glion in which secondary neurones originate. ^''^ 



Cephalopod hearts may also have accelerating fibers;"** in Eledone and Oc- 

 topus the auricles can be stimulated to high tonus, whereas the ventricle is 

 inhibited, and contraction can be elicited in a quiescent heart by stimulation 

 of a visceral nerve. A positive inotropic (increased amplitude) effect builds 

 up during repetitive stimulation,^^' ^^ and the threshold is higher for accel- 

 erator fibers than for inhibitory ones.^^** 



Acetylcholine inhibits the heart of all classes of molluscs (Table 68, 

 Fig. 206). Eserine fails to potentiate the nervous inhibition in cephalo- 

 pods,^^^ although it does prolong inhibition by visceral ganglion stimulation 

 in Venus}^^ Atropine is toxic and does not antagonize the inhibitory effect 

 either of visceral nerve stimulation or of acetylcholine.^-' ^■^*' Evidence for 

 a chemical mediator in Venus was obtained by inhibiting a test heart with 

 fluid from one inhibited by visceral ganglion stimulation. ^•^'' Perfusion fluid 

 from the heart of a cephalopod (Sepia, Octopus, Eledone) was passed into 

 the heart of another individual; when the excitatory fibers of the visceral 

 nerve of the donor were stimulated, the amplitude of the beat of the per- 

 fused isolated heart also increased, apparently because of a stimulating medi- 

 ator.'"'* Other investigators failed to confirm this experiment;'^"' also tests 

 for ACh in the perfusate from the hearts of Octopus and Eledone during 

 visceral nerve stimulation were negative. Identification of the substances 

 liberated by inhibitor and accelerator fibers in molluscan hearts would add 

 considerably to the knowledge of nerve function. 



The hearts of Crustacea and of Limulus receive from the central nervous 

 system several regulating nerves which are distributed to the pacemaker 

 ganglion and the muscle. Extrinsic cardiac innervation has been de- 

 scribed in detail by Alexandrowicz.^' -^ In 1896 Conant and Clark^** proved 

 by cutting and stimulation that the heart of Callinectes receives two pairs 

 of accelerator and one pair of inhibitor nerves. Cerebral ganglion stimula- 

 tion results in inhibition of the heart in crayfish, crabs, and Limidus.'^'^- *^- ^^- 

 In Limulus, the inhibitory nerves arise in the posterior part of the brain, in 



