THE KELATION OF THE HEART-BEAT TO ELECTROLYTES. 173 



in the environment in any experiment should be known with the 

 utmost precision. 



Practically every experiment involves the imposition of some ab- 

 normal conditions. In a state of nature, conditions are varying 

 continually ; the fixation of any condition in itself constitutes an 

 abnormality. After determining artificially one condition, it is 

 fallacious to speak of the remaining conditions as being " normal." 

 If they too are not experimentally controlled, they are unknown. The 

 choice lies, therefore, between a type of experiment in which most of 

 the conditions are unknown but where there has been but little 

 operative interference, and another type where the conditions are 

 simplified and controlled by artificial means. The former may yield 

 information of much value to the physician, but it is to the latter 

 that we must turn for advances in our knowledge of cell physi- 

 ology. 



If, now, we are concerned with the problem of how and why 

 the heart gives rhythmic contractions, the first step will be to deter- 

 mine and to define the simplest conditions under which the heart will 

 continue to beat. Eemoval of the heart from a freshly killed animal 

 suffices to show that the essential mechanism is self-contained. The 

 movements of the heart, though susceptible of control by the central 

 nervous system, are yet able to continue when all nervous connection 

 is severed, and indeed it is known that in the chick they begin in the 

 heart muscle some time before any nervous connection is established. 

 It might be suggested that the excised heart lying in a watch-glass 

 and continuing to beat was under the simplest conditions imaginable. 

 Very little consideration is needed to show that this is not the case. 

 In the first place, its temperature is not being controlled ; and secondly, 

 the heart muscle is in contact with a layer of fiuid of complex and 

 changing composition. As so often in scientific work, an elaboration of 

 apparatus is necessary in order 'to secure a simplification of experi- 

 mental conditions. That the composition of the fiuid in contact with 

 the muscle may be known as closely as possible it is desirable that a 

 large volume of the fluid should be available, and that the portion in 

 contact with the tissue shall be frequently renewed. For this purpose it 

 is not sufficient to immerse the heart, or portions of it, in a large vessel of 

 the solution. Such a method, it is true, has often been adopted, but it 

 is far from being satisfactory. The best method is to perfuse the heart 

 with fluid, letting the solution enter the venous end of the heart at a 

 small and constant pressure, and allowing it to escape at the aortae, 

 which may be opened up to allow of a free flow. In this way, in 

 the hearts of animals, where tlie coronary system has attained to no 



