BODY FLUIDS AND CIRCULATION 



117 



ceans and Limulus. These pharmacological observations are of interest 

 for the light they may shed on processes of cardiac regulation. The excita- 

 tory effect of acetylcholine resembles that produced by stimulation of 

 cardio-acceleratory fibres. The blocking and paralysing effects reported 

 for atropine and nicotine appear to take place in the pacemaker ganglion. 



/ mm. 



i 



1 mm. 



Fig. 3.19. Cardio-regulative Nerves of the Heart of Pamdirus argus 



(a). Effect of stimulation of cardio-inhibitor fibres on heart beat. Frequency of stimu- 

 lation varied (pulses per sec). (6). Effect of stimulation of cardio-accelerator fibres on 

 heart beat. Frequency varied (pulses per sec). (From Maynard (86).) 



A schema has been suggested ascribing cholinergic activity to extrinsic 

 acceleratory nerves and pacemaker (large intrinsic) neurones, and adrener- 

 gic activity to the intrinsic (small) motor cells (30, 31, 57, 75, 105, 115, 

 122, 133). 



Polychaetes. The contractile vessels of two polychaetes have been 

 investigated, namely Nereis and Arenicola. Nerve cells have been found in 

 the lateral hearts of Arenicola, but not in the contractile dorsal vessel of 



