March 19^ 1915] 



SCIENCE 



409 



action: automatic, axon reflex, uncondi- 

 tioned reflex, conditioned reflex and voli- 

 tional. It is not claimed that this list is 

 exhaustive. The physiology of such mech- 

 anisms, for example, as the plexus of 

 Auerbach is at present too little under- 

 stood to admit of successful classification. 

 It is only claimed that the above are dis- 

 tinct forms of nervous activity; that they 

 are carried out by different mechanisms, 

 and that, as such, they should be given co- 

 ordinate rank in the student's mind. 



In the following discussion I shall not 

 include any treatment of volitional action. 

 I am concerned only with the proper classi- 

 fication of nervous actions and see no rea- 

 son for changing the all but universal 

 custom of placing volitional actions in a 

 class by themselves. I shall, however, 

 dwell at some length upon the automatic 

 action, the axon reflex, and then discuss 

 together the conditioned and the uncon- 

 ditioned reflex. 



AUTOMATIC ACTIONS 



In general an automatic action is one 

 which originates in the mechanism in- 

 volved and is not caused by any external 

 influence acting only at the time of its oc- 

 currence. The ticking of a clock is an ex- 

 ample. In the field of physiology we think 

 at once of the beat of the heart, although 

 other no less striking examples are known ; 

 a strip of the muscular coat of the stomach 

 or intestine shows automatic contractions; 

 and many of the processes of embryolog- 

 ical development probably belong in the 

 same category. Contrasted with these are 

 skeletal muscles and many glands which 

 become functionally active only in re- 

 sponse to some sort of external stimulus, 

 usually a nerve impulse. 



Even in the case of the skeletal muscle, 

 however, the external stimulus seems to act 

 by causing the accumulation, and prob- 



ably the localized accumulation of some 

 physical or chemical condition within the 

 cell leading to the discharge of energy. 

 Thus an attractive theory of electrical 

 stimulation of skeletal muscle supposes 

 that certain semipermeable membranes 

 within the muscle fiber are more permeable 

 to ions of one electrical charge than to those 

 of the opposite charge; hence in the mi- 

 gration of the charges to the two electrodes 

 during the passage of a current there re- 

 sults an accumulation of electric charges 

 at these membranes; and when this has 

 gone on to a certain extent, the electrical 

 condition thus created explodes an unstable 

 fuel substance, energy is liberated and 

 contraction results. The passage of the 

 stimulating current has merely produced 

 the accumulation of what may be called 

 the "discharging conditions" within the 

 cell. 



In the automatic action these discharging 

 conditions seem to accumulate without ex- 

 ternal assistance, possibly as the result of 

 certain metabolic processes in the cell 

 itself. External conditions, such as tem- 

 perature, may influence the rate or the 

 amount of discharge; but this does not 

 make these external conditions stimuli in 

 any true sense. Furthermore, we may 

 speak, if we will, of the whole chain of 

 events leading to the accumulation of the 

 discharging conditions as an "inner stim- 

 ulus"; but this would seem to involve an 

 unnecessary and even questionable exten- 

 sion of the term stimulus. 



An automatic nervous action is fre- 

 quently defined as a discharge from a nerve 

 cell caused by some other external stim- 

 ulus than that of an exciting neurone ; but, 

 if the cell is discharged by an external 

 stimulus of any kind, the action is not 

 automatic. "What we observe in such eases 

 is activity, apparently, at any rate, arising 

 within the cell itself, and we have no more 



