Circulation of Body Fluids 547 



molluscs (Gastropdoa, Pelecypoda, Cephalopoda) (Table 68). Molluscan 

 hearts are very sensitive to acetylcholine, and inhibition of the heart of Venus 

 has been obtained with concentrations as low as 10~^^ ^^^- ^'*'' 



In general, the hearts of molluscs are very sensitive to pressure; it has been 

 claimed that some fail to beat unless they are distended."*^- '^^ It has also 

 been stated that small beats may occur in a collapsed heart. Certainly, when 

 the internal pressure increases, the beats are stronger and more frequent. 

 In Octopus vulgaris Fredericq"'^ recorded no beats at pressures below 2 

 cm. H2O, and the frequency was higher at an internal pressure of 85 mm. 

 H2O (59/min.) than at a pressure of 45 mm. H2O (52/min.). Also in some 

 clams the heart rate is accelerated while water is being pumped through the 

 mantle cavities. ^^^ The vertebrate heart also adapts to a given load so that 

 as the blood volume in the heart is increased the strength of the beat increases 

 (Starling's law). 



Neurogenic Hearts. The hearts of most crustaceans and of Limulus have 

 nerve ganglion cells on the dorsal surface which originate the excitation 

 wave for the heart beat. In crabs and crayfish the number of these large 

 ganglion cells is fixed. ■^- -"^ The hearts of higher crustaceans are accelerated 

 by acetylcholine, an effect opposite to that in vertebrates and molluscs 

 (Table 68; Fig. 198). Stimulation of the pacemaker ganglion of the crus- 

 tacean heart by acetylcholine is analogous to stimulation of sympathetic 

 ganglion cells by acetylcholine in vertebrates. In Artemia and Eubranchipus 

 acetylcholine is without any effect, even in high concentrations; these hearts 

 appear to be myogenic and non-innervated.^''^ 



The role of the dorsal gangHon is best shown in Limulus. The Limidiis 

 heart has eight pairs of ostia which serve to divide it into eight segments 

 (Fig. 198); in the anterior half are five pairs of arteries plus one antero- 

 median artery. -^"^ Lateral trunks of nerve fibers run along each side and are 

 connected by thin nerve bundles to the median ganglionic trunk. This gan- 

 glion contains scattered small multipolar nerve cells and large unipolar 

 nerve cells, the latter located mostly in segments 4 and 5. Local elevation 

 of temperature, as by applying a warm test tube or local electrical stimula- 

 tion, alters the heart rate most when applied to the fourth and fifth segments 

 of the ganglion. Normal heart activity begins first in the muscle adjacent 

 to the ganglion in the fourth and fifth segments. If the dorsal ganglion is 

 removed, the heart stops beating. If the heart is then placed in NaCl solu- 

 tion, -^^^ ■^■' or if tension is applied as by inflating the heart, there may be 

 some contractions; these contractions of the deganglionated heart are slow, 

 and may be local or peristaltic, hence they are not normal beats. In Lirmdus 

 embryos a heart beat (peristaltic) begins at 22 days, but no nerves are pres- 

 ent until the 28th day.^*" Drugs afl^ect the heart difi^erently before and after 

 innervation;^^^ although the adult heart is accelerated by acetylcholine, the 

 beat of the embryonic heart prior to innervation is unaffected. Evidence 

 from action potentials, to be presented below, agrees with the above data, 

 that the large ganglion cells of the mid-segments originate the excitation 

 wave and relay it by way of the smaller neurones out to the muscle. Con- 

 duction of excitation is by the median and lateral nerves. When the median 

 nerve cord is cut away, bit by bit, the amplitude of the contractions dimin- 



