Circulation of Body Fluids 559 



Crustacea both inhibitor and accelerator nerves arise from the subesophageal 

 ganglion, the inhibitors always arising anterior to the accelerators. The sto- 

 matogastric nerves have no effect on the heart. -"•'*• i**- In Astacus the inhibi- 

 tor nerve fibers elicit maximal inhibition at a stimulation frequency of 45/sec. 

 and accelerators are most effective at 30/sec. In the grasshopper, stimulation 

 of either brain or ventral ganglia results in augmentation of heart beat. 



Acetylcholine accelerates the neurogenic hearts of arthropods, and the effect 

 resembles qualitatively stimulation by cardioaccelerators (Fig. 209). Eser- 

 ine has been said to enhance nervous acceleration but not nervous inhibition 

 in Astacusr^-' and to have no effect on either acceleration or inhibition in the 

 crab Cancer. "^^'-^ Atropine antagonizes the effect of acetylcholine but has 

 no effect on nervous control. There is no convincing evidence that either 

 accelerator or inhibitor nerves in Crustacea are cholinergic. In Daphnia 

 mechanical stimulation of the intestine reflexly inhibits the heart and recov- 

 ery of the beat is hastened by acetylcholine,^^^" although under other condi- 

 tions acetylcholine may decrease the heart rate.^'' In the shrimp Palaemonetes 

 a hormone from the sinus gland accelerates the heart.^'^'"' 



The only record of extrinsic nervous effects on the hearts of annelids is for 

 Nereis and Arenicola; stimulation of the ventral nerve cord inhibits the 

 lateral hearts in diastole and increases the rate and strength of dorsal vessel 

 contractions.'**' Earthworm vessels continue to beat after destruction of the 

 ventral nerve cord.^*^"- ^ 



Nervous control of the heart of ascidians has not been clearly demonstrated. 

 Removal of the "brain" from Ciona is said not to affect the heart,^" although 

 in Molgula the beat is made irregular. '*'' In Ascidia mentida removal of the 

 "brain" results in faster heart rate for about a day.'"' 



Nervous control of cardiac activity is found in most animals. Only in em- 

 bryos (vertebrate, Limidus') and possibly in certain lower crustaceans, such 

 as Artemia and Euhranchipiis,^^'' is there evidence that nervous regulation is 

 absent. In general, chemical mediators associated with activity of cardiac 

 nerves are indicated. Acetylcholine inhibits the myogenic hearts of verte- 

 brates and molluscs; it accelerates the neurogenic hearts of Crustacea, Limulus, 

 and some insects and annelids. Adrenalin accelerates all hearts. Evidence 

 for cholinergic regulating nerves is available only for vertebrates and for 

 Venus. Possibly undiscovered agents are active as mediators in some inverte- 

 brates. 



DIRECT RESPONSES OF HEART TISSUES 



Effects of Drugs on Hearts. The literature on the pharmacology of circu- 

 latory systems is extensive and is useful for comparative physiology m so far 

 as it aids in an understanding of hearts and peripheral circulation. The in- 

 hibiting and accelerating actions of ACh are illustrated in Figures 207-210; 

 these effects have been discussed in the sections on pacemakers and nervous 

 regulation of hearts. The action of drugs which potentiate or antagonize 

 acetylcholine indicates interesting differences in cardiac mechanisms. 



Physostigmine (eserine) and prostigmtn^ prevent hydrolysis of acetylcho- 

 line by acetylcholine esterase; in the vertebrates they prolong the inhibitmg 

 effect of acetylcholine and of vagal stimulation. Eserine sensitizes hearts to 

 ACh. Potentiation of the inhibiting effect of ACh has been reported in the 



