STIMULATION OF LIVING ORGANISMS 581 



the latter a process or series of processes in which the transformation of 

 energy may be almost indefinitely large — out of all proportion to the 

 exciting stimulus. We have therefore first to inquire into the general 

 nature of the conditions that render possible such disparity between the 

 stimulus — considered by itself as a particular chemical or physical pro- 

 cess acting upon the irritable tissue — and the resulting special activity 

 or response on the part of the tissue itself. 



As all know, an irritable tissue like a nerve or muscle may be aroused 

 to activity under the most various conditions ; the effective stimulus may 

 be an electric shock, a chemical substance, the action of light, change of 

 temperature, loss of water, mechanical impact; and the tissue gives the 

 same response to all of these. Now it is clear that such a stimulus can 

 only act as some kind of a releasing agency — what Ostwald calls Anlass — 

 which sets going some process all of the necessary conditions of which 

 are already present, but which is held in check by some restraining con- 

 dition which the releasing agency removes — as when a gun is fired, or an 

 alarm-clock set olf, or a mine exploded by the pressure of a button — 

 which closes an electrical circuit, thus enabling a spark to pass, which 

 raises the temperature of the explosive to the critical point. The connec- 

 tion between Anlass and resulting event may be highly indirect, and 

 there need be no resemblance or other relation than that of interconnec- 

 tion between the two. In all cases the system is, as it were, "wound 

 up"; the potential energy is there, ready to become kinetic; once the 

 process is started or activated by the releasing event, it proceeds of its 

 own accord to its conclusion, i. e., till a second state of equilibrium is 

 reached. In the case of a living irritable tissue or organism we are evi- 

 dently dealing with a physico-chemical system belonging — as regards the 

 relations between the initiating conditions and the resulting process 

 itself — to this general class. If we press the end of a nerve connected 

 with a muscle, or pass through it an electrical current of sufficient inten- 

 sity for a sufficient length of time, or dip it into a solution of some 

 appropriate chemical substance, there is initiated at the site of stimulus 

 a " physiological " process which is propagated with unaltered intensity 

 along the nerve to the muscle and there calls forth a complex variety of 

 interdependent physical and chemical changes, of which the contraction 

 is the most conspicuous and physiologically important. Thus a process 

 specific to the tissue, unique and obviously highly complex, is initiated by 

 the relatively insignificant change which the stimulus causes directly. 

 We ought particularly to note that in any special tissue the physiological 

 process remains the same in kind, whatever the nature of the stimulus. 

 The latter merely causes some critical or releasing change which initiates 

 the physiological sequence of events; the latter then proceeds automati- 

 cally in its characteristic way to its conclusion. 



Let us now consider more particularly the physiological part of the 



