



10 



Microns 



Figure 221. — Nerve cells in the heart muscle fiber of C. virginica. Methylene blue vital stain. 



Pharmacological evidence of tlie effect of drugs 

 on heart, described later (p. 252), and particularly 

 the action of acetylcholine and the antagonism of 

 curare to acethycholine, support the view that 

 the pacemaker system in the oyster heart is of a 

 diffuse mj^ogenic nature. 



METHODS OF STUDY OF HEART BEAT 



In order to count the number of beats per unit 

 of time a portion of the left valve must be removed 

 without injury to the adductor muscle and tlie 

 underlying tissue. The oyster is then kept in 

 sea water at constant temperature, and tiie num- 

 ber of beats is recorded. The method was used 

 by Federighi (1929) and by Koehring (1937), 

 who drilled a small round window in the valve 

 and with sharp scissors dissected the pericardium 

 to expose the heart. These oysters lived for several 

 weeks in running sea water in the laborntory of 

 the Bureau of Commercial Fisheries at Woods 

 Hole without noticeable ill effects. 



Stauber (1940) modified the technique by 

 cutting windows in both valves without injury to 

 the pericardium wall and cementing them over 

 with pieces of glass or cellophane. For observa- 

 tion the operated oysters were illuminated from 

 underneath. In a few days both were covered by 

 new shell and had to be replaced. Shell material 

 that covered the window of the left side, where the 

 pericardium wall touched the valve, probably 

 spread from the adjacent areas of the mantle. 



Figure 222. — Electrocardiogram of C. virginica tal^en in 

 situ. A gentle wave corresponding to auricular con- 

 traction A precedes by approximately one-half sec- 

 ond the contraction of the ventricle. Temperature 22.(3 

 °C Time intervals, 1 second. 



Pulse records can be obtained without touching 

 the heart itself by removing a portion of the valve, 

 using the pericardium wall as a sphygmograph 

 tambour, and providing a small stand made of 

 light plastic to support one arm of the writing 

 lever. The disadvantage of this method used in 

 the shellfish laboratory at Woods Hole was that 

 the heart became fatigued after several hours of 

 recording. 



There is another technique to study heart 

 contraction in situ. The pericardium wall is 

 exposed by cutting off the valve above the adductor 

 muscle. A small S-shaped glass hook connecting 

 the heart with the kymograph lever is placed 

 under the auriculo-ventricular junction or under 

 the ventricle. A silk thread tied to the upper 

 part of the hook is connected to a writing lever, 

 which is carefully balanced so that the tension on 

 tiie heart does not exceed 100 mg. Care must be 

 taken to adjust the tension so that the pull of the 

 hook will not displace the heart from its normal 

 position (fig. 2215). 



There will be a minimum of damage to the 

 nervous system and adjacent organs if only part 

 of the vah'e between the adductor muscle and the 

 hinge is removed. This leaves the muscle itself 

 intact, and only the pericardium wall is dissected 

 to expose the heart. The oyster is kept in a 

 known volume of water in a finger bowl, which is 

 placed in a large crystallizing dish to permit the 

 rapid change of water or of experimental solution 

 without disturbing the setup. Temperature in 

 the larger dish (not shown in figure 223) is 

 thermostatically controlled at any desired degree. 

 Under such conditions the beating of the heart 

 continues for about 2 days. 



The perfusion chamber method is frequently 

 employed (fig. 224) in the pharmacological 

 studies of the effects of drugs on bivalve hearts. 

 In this method tiie heart is cut off at tlie levels of 

 the auricles and the aorta, ligatures are applied at 



CIRCULATORY SYSTEM AND BLOOD 



247 



