410 



SCIENCE 



[N. S. Vol. XLI. No. 1055 



logical right to assume an unrecognized 

 stimulus than in the case of the ticking of 

 a clock. In the absence of knowledge to 

 the contrary, the presumption is that the 

 cell is discharged by the operation of its 

 own never ceasing metabolism. 



In our usual teaching as well as in our 

 usual thinking it is customary to take ac- 

 count only of (1) the stimulus and (2) the 

 reaction, and to regard each of these as a 

 single process; whereas all the evidence 

 goes to show that between the ordinary ex- 

 ternal stimulus, at any rate, and the re- 

 lease of energy there is usually interpo- 

 lated a third process, which we have termed 

 the "accumulation of discharging condi- 

 tions" in our brief reference to the nature 

 of electrical stimulation. We again meet 

 with the suggestion of a similar process in 

 the ease of stimulation by a nerve impulse. 

 Langley's work on the antagonism of nieo- 

 tin and curare, as well as that of Keith 

 Lucas on the "characteristic" of stimu- 

 lation in different tissues, has led to the 

 assumption of a "receptive substance" in 

 skeletal muscle. The action of adrenalin 

 also points to a similar "receptive sub- 

 stance" connected with the endings of the 

 post-ganglionic automatic neurones. And 

 yet is not this term "receptive substance" 

 or "receptor" merely a name to hide our 

 ignorance? and do we not really mean a 

 physical or chemical process carried out 

 by the cell protoplasm, as a whole, rather 

 than a specialized irritable substance; a 

 process, in other words, which results in the 

 accumulation of ' ' discharging conditions ' ' ? 

 If this point of view is correct we must dis- 

 tinguish sharply between the stimulated 

 action and the automatic action; they are 

 alike in the second and third of the above- 

 mentioned processes; they differ in that 

 the accumulation of discharging condi- 

 tions comes in the stimulated action as the 

 result of an external influence (electric 



shock, nerve impulse, or mechanical blow), 

 while in the automatic action it results pre- 

 sumably from the cell metabolism. 



In the central nervous system the best 

 known and most successfully studied case 

 of automatism is that of the respiratory 

 center. The conclusion which Eosenthal 

 drew from his experiments, that the nerve 

 cells of this center send out rhythmic dis- 

 charges when removed from all connection 

 with afferent nerves, has been confirmed by 

 all subsequent work, the experiments of 

 "Winterstein being especially conclusive on 

 this point. Here again we are probably 

 dealing with the development within the 

 nerve cell of discharging conditions which 

 may be influenced by the character of the 

 environment, such as the tension of car- 

 bon dioxid or the concentration of hy- 

 drogen ions, or temperature or the pres- 

 ence of certain drugs; and we may repeat 

 that there is no justification for speaking 

 of these as stimuli, as we generally do. So 

 far as the facts go, we may logically re- 

 gard them only as external conditions 

 which regulate the rate of development of 

 the automatic cell processes or the charac- 

 ter of the discharge which it evokes from 

 the cell. 



Until recently no other case of automatic 

 nervous action was known. Some may 

 have been suspected in the vaso-motor sys- 

 tem or in the myenteric plexus; but no 

 facts compelled the conclusion that they 

 must be regarded as automatic actions. 

 Recently, however, facts have come to light 

 which argue strongly for an automatic 

 basis to the nervous mechanism of locomo- 

 tion. These movements, as already stated, 

 have in the past presented to us the pic- 

 ture of an endless chain of reflexes, in 

 which the afferent neurones are mostly the 

 nerves of muscular sense, the complex of 

 instreaming afferent impulses, ever chang- 

 ing as the movement proceeds, giving the 



