BODY FLUIDS AND CIRCULATION 119 



Nereis (Neanthes). There appear to be avenues for extrinsic nervous con- 

 trol, since stimulation of the ventral cord inhibits the lateral hearts of 

 Arenicola, and increases the rate and amplitude of contractions of the 

 dorsal vessel in Nereis. It appears from the electrocardiogram that the 

 heart beat of Arenicola is neurogenic in origin. The electrical response 

 shows an initial rapid negative wave, followed by a positive deflexion, 

 often with several superimposed oscillations. The oscillations and the fast 

 spike would appear to represent the summated discharge of ganglionic 



Fig. 3.21. Effects of Drugs on the Isolated Heart of Cancer magister 

 {Left) acetylcholine, 1 : 100,000. (Right) nicotine, 1 : 10,000 (From Davenport (30).) 



pacemakers in a neurogenic heart. Acetylcholine has an acceleratory and 

 augmentor action on the heart of Arenicola, and the effects are increased 

 by eserine (107). 



Tunicates. The nature of the pacemaker mechanism in the heart of 

 tunicates has not been resolved. Nerve cells appear to be absent, although 

 some uncertainty exists about this point since the hearts are rather refrac- 

 tory to methylene-blue staining methods. The heart-beat normally origi- 

 nates at one or other end of the heart, which are pacemaker regions, but 

 even isolated middle pieces of the heart of Molgula will continue to beat 

 for some time. The heart regularly reverses the direction of beat, pace- 

 maker centres at each end of the heart taking control alternately. To 

 explain this regular reversal of control it has been argued that a back 

 pressure is gradually built up in the circulatory system, compelling the 

 heart to stop, and allowing the opposite centre to take over. Against this 

 it is pointed out that completely isolated hearts show periodic reversal in 

 the absence of back pressure: a controlling mechanism for reversal inher- 

 ent in the pacemaker is thus implied. Krijgsman suggests that reversal is 

 due to periodic fatigue (adaptation) followed by recovery, alternately, in 

 the pacemakers (36, 59, 76, 89). 



IONIC EFFECTS AND BALANCED SALT SOLUTIONS 



Like all excitable tissues the heart is dependent on a proper ionic balance 

 in the fluids surrounding it for optimal activity. Hearts will continue to 

 beat for many hours outside the body when suspended in properly balanced 



