STUDIES ON THE CARDIAC MUSCLE OF LOWER VERTEBRATA

Both in the heart in situ and in the excised peftused heart great variation may occur in the rate of recovery after a contraction and in the position and duration of the phase of maximum responsiveness. Early‐responsiveness may or may not be followed by early decline. Staircase of contractions after a prolonged pause depends on a decline in responsiveness occurring during the standstill and being removed by serial excitations. Staircase is ( a ) promoted by acidity, vagus influence, pilocarpine, chloroform, excess of potassium, defect of calcium, and by sodium cyanide at a certain phase, and ( b ) opposed by adrenaline, reduction of sodium chloride, digitaline, excess of calcium, strontium, etc. Staircase may occur with strongly alkaline perfusion. Staircase may result from a premature contraction which is not followed by the usual elongated pause ( e.g . under the influence of NaCN). There is a depressing effect extending over two or three beats in contrast to the improving effect which a premature contraction may exercise under certain other conditions. Enlargement of premature beats from a previously depressed level towards normal size (the so‐called supernormal contraction) may occur in two conditions: ( a ) as the beginning of a staircase, and ( b ) where contraction of full size is at once set up by excitation after an optimal interval, immediately weakening again after a non‐optimal interval. In ( a ) the fact of activity is the essential influence, leading to progressive augmentation; in ( b ) the length of the interval determines the sudden attainment of full strength. The supernormal condition can be induced by numerous agencies besides acidity, and there are considerable variations in frogs under apparently similar experimental conditions. The relations of the position and size of the premature beat with regard to the ordinary or primary beat can be greatly affected by adrenaline, cyanide, chloroform, pilocarpine, stoppage of perfusion, etc. In contrast with their ordinary features, premature contractions may, under certain conditions, show a large and long mechanical response with long refractory period, low threshold, and short latency. The occurrence of a premature contraction may lead either to augmentation or to depression of the following beat; these changes can be brought about in different ways. Some excessively long latencies of premature contractions that have been described are not true ventricular latencies, but are mediated through supra‐ventricular tissues; they are not seen in the isolated ventricle. Superposition of contractions can be produced under various conditions, almost constantly with NaCN, NaC1‐lack, digitaline; less regularly with chloroform, pilocarpine, chloral, urethane, acidity, and increased potassium. Complete tetanus resembling that of skeletal muscle can be obtained by electrical stimulation under the influence of NaCN, digitalis, and reduction of NaC1. There are certain differences in the behaviour of the ventricular muscle of the eel and tortoise as compared with the frog. There are notable resemblances between many of the results described in this paper and those described in the mammalian heart by MacWilliam , as regards premature contraction, superposition, staircase, optimal rhythm, etc. I wish to thank Professor MacWilliam for his advice and help in the course of this investigation. The work was carried out during the tenure of a Carnegie Teaching Fellowship, and a grant in aid was received from the Carnegie Trust.