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Figure 227. — Three records of heart beat of C. virginica 

 in situ. The upper curve was obtained by placing the 

 connecting hook of a kymograph lever under the ven- 

 tricle. The two lower curves were made when the hook 

 was placed under the auriculo-ventricular junction. In- 

 creased frequency of the lowest curve is associated with 

 an increase of temperature of sea water from 20.5° to 

 24.5° C. Time interval, 2 seconds. 



celerating and inhibiting nerves lead from the 

 visceral ganglion to the heart and that the two 

 other ganglia affect the heart by way of the visceral 

 ganglion. Oka (1932) found that stimulation of 

 the visceral ganglion inhibits both auricular and 

 ventricular rhythms, and Irisawa, Kobayashi, and 

 Matsubayashi (1961) determined the action po- 

 tentials in 0. laperousi and found that oyster 

 heart rela.xes through anodal current. 



The cardiac nerve is a small branch of the 

 visceral nerve which emerges from the cerebro- 

 visceral connective near the x-isceral ganglion. Its 

 branches enter the auricles at their base and 

 regulate only the auricular rhythm. The ventri- 

 cular rhythm, according to Oka's view, is regu- 

 lated by the cardiac nerves which enter the 

 ventricle at tlie aorta end. This finding is not in 



Figure 228. — Tracings of the beating of the excised heart 

 of C. virginica at 20° C. Salinity 31.7 °/oo. Time 

 interval, 5 seconds. 



agreement with Carlson's observations that the 

 cardiac nerves enter the heart of a bivalve at 

 the base of the auricles and not at the aortic 

 end. Experimentation with the oyster heart is 

 difficult because exposure of the ganglion causes 

 profuse bleeding and collapse of the heart. Ftu-ther- 

 more, the cardiac nerves in C. virginica are ex- 

 tremely small and difficult to observe in the living 

 tissue. 



Investigations by Carlson did not demonstrate 

 the presence of acceleratory nerves in the hearts of 

 bivalves. Oka (1932) thinks that possibly both 

 kinds of nerves, the acceleratory and the inhibitory, 

 are present in the heart of 0. circumpicfa but that 

 the action of the inhibitory nerve predominates. 

 The suggestion is based on the observation of old 

 heart preparations of lowered vitality in which the 

 beat of the auricles was slightly accelerated by 

 stimulation of the ganglion. The evidence is not 

 con\'incing and requires verification. 



Krigsman and Divaris (1955) arrive at the 

 following conclusions which appear to be applicable 

 to the oyster heart: 1) The systolic mechanism 

 is situated in the heart's muscle fibers; and 2) 

 extrinsic regulatory nerves infiuence the pace- 

 maker system. The inhibiting fibers are probably 

 cholinergic, and the accelerating fibers may have 

 adrenergic properties. The latter statement needs 

 further verification. 



EFFECTS OF MINERAL SALTS AND DRUGS 



Bivalve he^irts respond readily to changes in 

 the chemical composition of water and to the 

 presence of low concentrations of various drugs 

 and poisons. Because of this sensitivity the hearts 

 of several common species such as Anodonta, Mya, 

 Mercenaria, Ostrea, and others often have been 

 used in pharmacological bioassays. The test is 

 usually made with a preparation of an excised 

 entire heart (or ventricle) in the perfused chamber. 

 Increased acidity slows the beat of the excised 

 heart of C. virginica; a pH of 4.0 and lower 

 causes diastolic arrest and from pH 4 to 9 the 

 rate increases with the increase of pH values. 

 Above pH 9 the contractions become irregular 

 (Otis, 1942). 



A change in the balance of metallic ions in the 

 surrounding water affects cardiac activity. Small 

 excesses of potassium stimulate the heart by in- 

 creasing the frec|uency of beat (positive chrono- 

 tropic effect) and by changing the tonus (tono- 

 tropic effect) of the myocardium (.Tullieii and 

 Morin, 1930, 1931b). 



CIRCULATORY SYSTEM AND BLOOD 



251 



