Circulation of Body Fluids 557 



superimposed (Fig. 206). It is probable that the excitatory wave originates 

 in these large cells and that their slow potentials electrotonically synchronize 

 the discharge of the smaller multipolar cells. 



Electrocardiograms show three components: (1) "fast" waves associated 

 with propagation, (2) "slow" waves probably associated with contraction, 

 and (3), in neurogenic hearts, oscillations corresponding to the pacemaker 

 discharge. 



Nervous Regulation of Hearts: Cholinergic and Adrenergic Systems. 

 The vascular demands for blood vary with activity and stress. The heart 

 accelerates or decelerates reflexly in efficient vascular systems. The higher 

 vertebrate brain (medulla) contains a vasomotor center which receives im- 

 pulses from: (1) specific vascular receptors by way of the carotid sinus, and 

 depressor and vagus nerves, (2) general somatic and visceral receptors, and 

 (3) higher centers in the brain. The heart rate rises in compensation for a 

 fall in blood pressure, and the heart slows when the blood pressure rises. 

 The vagus nerves slow the heart and reduce the amplitude of contraction; 

 the primary vagus fibers terminate in ganglia located in several regions of 

 the heart,-"-' and secondary vagus neurones end in the heart musculature 

 and nodal tissue. Secondary sympathetic fibers cause cardio-acceleration. 

 In addition, adrenalin added directly to the heart accelerates it, but in the 

 circulation of the intact animal reflex control usually prevents marked accel- 

 eration on injection of adrenalin. The embryonic heart beats for some days 

 before innervation; during this early period reflex regulation of heart rate 

 cannot occur.' 



Activity in the sympathetic fibers to the heart is associated with liberation 

 of sympathin, an adrenalin-like substance. Sympathetic innervation of the 

 heart is lacking in fishes, but vagal fibers are found in all classes of verte- 

 brates, with the possible exception of cyclostomes.-^- ^^■* Reflex inhibition of 

 the heart of dogfish is obtained by stimulation of a variety of sensory path- 

 ways. Inhibitory vagal reflexes can be elicited in the elasmobranch Scyllium 

 by afferents in the vagus, hypobranchial, and lateral line nerves.^-'^ Altera- 

 tion of water flow over the gills affects both respiration and heart beat re- 

 flexly. The inter-renal glands of fish are well developed, and adrenalin in- 

 creases blood pressure but has no eff'ect or is slightly inhibitory on dogfish 

 hearts."^^ 



Acetylcholine (ACh) appears in perfused hearts during vagal inhibi- 

 tion.^-"' ^-■^ Atropine blocks the inhibiting action of the vagus, and the 

 drug physostigmine (eserine) enhances the effect. Embryonic hearts beat 

 for some days before innervation; during this early period reflex inhibition 

 of the heart does not occur; the heart is relatively insensitive to ACh. It 

 is probable that the ACh acts at the synapses of the vagus rather than on the 

 heart muscle. •"* Whether the secondary vagal fibers are cholinergic is not 

 known, but, by analogy with postganglionic parasympathetic fibers in the eye, 

 they may liberate ACh. Perfusion of very small amounts (10"^") of acetyl- 

 choline may increase contractile tension of mammalian hearts.^'^*^ 



Reflex regulation of hearts is of general occurrence among all phyla of 

 animals possessing hearts. The early literature has been excellently summar- 



