The pathway of the median giant fiber in the crayfish cephalic ganglion(brain) was reconstructed from serial sections. The median giant fiber in the para-esophageal nerve runs anteriorly and enters the brain. After turning medially, i t gives off a branch which further divides into two branches. These brsnches run ventrally and outwards and finally enter the ipsilateral antennule nerve and the ipsilateral antennary nerve, respectively. The main fiber runs medially and ventrally. It decussates and makes contact with the contralateral median giant fiber near the midline. After decussating, the fiber again divides into two branches. One runs outwards and ventrally and finally enters the contralateral antennule nerve. The other runs anteriorly at first and then turns ventrally immediately behind the rostra1 cellular group. Near the ventral surface of the brain the fiber turns posteriorly and connects with a large soma 100-150 p in diameter. In its course to the soma, the main fiber decreases in width to oneeighth of the diameter of the axon in the para-esophageal nerve.

In order to examine the pH dependency of Cd effects on cardiac muscle, the electrical and mechanical activities in the bullfrog heart were investigated in relation to various external pHs and concentrations of Cd. The amplitude and duration of the action potential or the spontaneously beating frequency in the atrium were not significantly affected during the 3-min administration of various concentrations of Cd in a range from pH 6 to 10, but the isotonic contraction at the end of the same period was greatly altered: the lower the external pH the more Cd decreased the contractility of the atrium. This pH dependency of Cd effects was also observed in SO4-Ringer's solution whose anions, SO4, were more impermeable than Cl. Decrease in contractility in CdRinger's solution was counteracted by excess Ca. The lower the pH of the Cd-Ringer's solution, the more Ca was necessary to counteract an equal amount of Cd. The amount of Cd-uptake into the atrium was analyzed after soaking the atrium in various concentrations of Cd-Ringer's solution. The higher the concentration of external Cd, the larger the Cd-uptake into the atrium. No pH dependency, however, was observed in this relationship. This suggests that probably only a small fraction of total Cd-uptake interacts with Ca-binding sites which is specific to contraction, and causes the Cd-induced decrease in contractility. This process is considered to be pH-dependent. However, most Cd-uptake into cardiac muscle is pH-independent and may be nonspecific to contraction.Effects of cadmium (Cd) on the electrical and mechanical activities of cardiac muscle in normal pH solution have been studied (KLEINFELD et al., 1955;TAKAHASHI et al., 1956;KLEINFELD and STEIN, 1968;HAYASHI and HORIUCHI, 1971a;TODA, 1973). It was found that a low concentration of Cd (about 10 /AM) brought about a decrease in contractility without an appreciable change in the action potential, and this Cd effect on contractility was completely antagonized by excess Ca (HAYASHI and HORIUCHI, 1971a). It was also reported

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Chemical analysis on the cadmium content in various organs disclosed that trace of cadmium was found even in the hearts of some healthy animals and men1,2) . As a large amount of cadmium was found in the heart of patient of chronic cadmium intoxication3) cadmium is considered to have high affinity to some structural components of the hearts . On the other hand, it has been suggested from the authors' electrophysiological experiments on the contractility of the frog heart that Cd ions may competitively occupy the marginal zone of the cardiac muscle fiber where Ca ions normally exist" . The purpose of the present paper is to demonstrate histochemically the binding of Cd with the frog cardiac muscle fiber.The hearts of bull frogs (Rana catesbeiana) were excised out immediately after intrathecal urethane anethesia (1g per kg body weight) . The hearts were divided into two or four strips along the heart axes . One halves of the hearts were soaked into Ringer's solution (NaCl 110 mM, KC1 2 mM, CaC12 1 mM, glucose 10 mM, tris buffer 10 mM, pH 7.2) containing various concentrations of Cd and the other halves of the hearts were soaked into Ringer's solution without Cd as control . Macroscopic observationBoth halves of the frog hearts pretreated with and without Cd were soaked in a pH4 .0Ringer's solution (phthalate buffer) saturated with H2S for 10 minutes and were fixed in pure ethanol for more than 10 minutes. H2S reacted with Cd and yellow cadmium sulfide precipitates were formed in the tissue. Fig. 1 illustrates the macroscopic color difference between the control half (right) and Cd pretreated half (left) : the Cd pretreated heart ( Fig. 1-A) was more yellowish than the control (Fig. 1-B) distinctively . The color difference of staining between 0.1 mM Cd pretreated half and control half was discernible . These macroscopic observations suggested that Cd might bind with some structures of both atri um and ventricle. Microscopic observationSeveral histochemical techniques" were applied to the sectio ns of the hearts in order to examine the physiological assumption that cadmium binds with ca rdiac muscle fiber.The conditions (pH, concentration of staining solution) for chelati ng to obtain the largest *Present address: Department ot rhysiology,

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