492 GENERAL PHYSIOLOGY 



the two metabolic series, it is evident that at present we are quite 

 unable to review even approximately the special changes that 

 biotonus experiences in concrete cases under the action of a 

 stimulus. For the present it is only possible to analyse step by step 

 the outward expression of these changes, which have been termed 

 schematically phenomena of excitation, depression and metamor- 

 phosis. Physiology will draw nearer to the solution of this problem 

 the more the methods of cell-investigation are developed. 



2. The Interference of Reactions 



The question of the effects of the interference of two different 

 stimuli is of special interest with reference to a group of very im- 

 portant phenomena belonging to the special physiology of verte- 

 brates. Unfortunately up to the present time there has been no 

 systematic treatment of this subject, and it is only possible to 

 present a few intimations of its connection with certain facts from 

 widely separate physiological fields. 



Since biotonus can be inflilenced very differently by different 

 stimuli, according as this one or that one of its components is 

 excited or depressed, in a systematic investigation of the effects of 

 the interference of two stimuli the manner of action of each must 

 form the starting-point. In order to understand any such effect 

 it must first be decided whether or not the two stimuli act in the 

 same manner, i.e., to excite or depress, and to what component of 

 biotonus their action extends, assimilation or dissimilation. The 

 general laws of interference-effects can be discovered only by 

 answering these questions. 



If two stimuli of medium intensity produce effects of the same 

 kind, for example an excitation, and act upon the same components 

 of biotonus,' for example upon dissimilation, the general result will 

 be a summation of the excitations. The details of this cannot be 

 predetermined, because the intensity of the stimuli, the varying 

 extent to which the individual components are influenced, the 

 duration of the stimuli, the fact of the self-regulation of metabolism, 

 etc., are factors which, under the circumstances, are capable of 

 playing important roles in bringing about the final result. Here 

 belongs, for example, the whole vai'iet}' of phenomena that we have 

 become acquainted with in nerve and muscle physiology as cases 

 of increase of irritability. Through the action of an exciting 

 stimulus, such as a chemical or thermal stimulus upon a nerve, the 

 irritability of the latter toward a second, such as a galvanic stimulus, 

 is increased, and the latter causes a greater reaction than if it had 

 been employed alone. 



A contrast to this is afforded by the phenomena that result when 

 living substance is acted upon by two stimuli that work in opposite 

 senses upon like components of biotonus, one depressing and 



