198 -ANIMAL PHYSIOLOGY. 
that there must be a point at which it is not experienced (indif- 
ferent point or neutral point). 
Now, it is possible to understand why a sudden change in 
the current should cause a muscular contraction. An equally 
sudden change, a profound molecular effect, has been caused, 
and this we must believe essential to the causation of a muscu- 
lar contraction through the influence of a nerve. 
To use an illustration which may serve a good purpose if 
not taken too literally, it is a well-known experience that one 
sitting in a room in which a clock is ticking soon fails to notice 
this regular sound; but should the clock stop suddenly or as 
suddenly commence to tick very rapidly, the attention is 
aroused, while a very gradual slowing to cessation or the re- 
verse would have escaped notice. The explanation of such 
facts takes us down to the very foundations of biology; but 
just now we wish only to elucidate by our own experience 
how it is possible to conceive of a muscle being stimulated 
by the molecular movements of nerve, or rather a change in 
these. 
There are important practical aspects to this question. One 
may understand why it is that electricity proves so ready a 
stimulus, and is so valuable a therapeutic agent. It seems, 
in fact, as will be learned later, to be capable of taking the 
place to some extent of that constant nerve influence which 
we believe is being exerted in the higher animals toward 
the maintenance of the regularity of their cell-life (metabol- 
ism), 
Pathological and Clinical.—It is believed that in the nerves of 
man, within his living body, the electrotonic condition can be 
induced as in an isolated piece of nerve. Hence, the value of 
the constant current in diminishing nerve irritability in neu- 
ralgia and allied conditions. Apparatus of great nicety of con- 
struction and capable of generating, accurately measuring, and 
conveniently applying electrical currents of different kinds, 
now adds to the resources of the physician. But we are prob- 
ably as yet only on the threshold of electro-therapeutics. 
Law of Contraction (Stimulation).—A given piece of nerve is 
stimulated only by the appearance of catelectrotonus, and the 
disappearance of anelectrotonus; but the disappearance of cat- 
electrotonus and the appearance of anelectrotonus are without 
effect (Pfliiger). This so-called law is supposed to explain the 
following facts, which may be thus expressed in tabular form 
(after Landois) : 
