APPLICATIONS OF THE GRAPHIC METHOD. 185 
cular contractions occurring within the living body are tetanic 
—i, e., the muscle is in a condition of shortening, with only very 
brief and slight phases of relaxation; and that a comparatively 
small number of individual contractions suffice for tetanus 
when caused by the action of the central nervous system; 
though, as proved by experiments on muscle removed from the 
body, they may be enormously increased. While a few stimu- 
lations per second suffice to cause tetanus, it will also persist 
though thousands be employed. 
The Strength of the Stimulus— We have assumed that in the 
cases of contraction thus far considered the stimulus was ade- 
quate to produce the full amount of contraction, or as much as 
could be obtained. Such a contraction and such a stimulus are 
spoken of as maximal; but the stimulus might fall a little 
short of this, and is then termed sub-maximal ; or it may be re- 
garded from the point of view of being the least that will cause 
a contraction, and is then the minimal stimulus. 
It is important to note that any sudden change in an electric 
current will act as an excitant to muscular contraction, but 
that very considerable changes in the strength of the current if 
made gradually do not react on the muscle. It sometimes hap- 
pens that a sudden onward push of the secondary coil of an 
induction-machine will produce either a tetanus (though the 
terminal wires or electrodes were arranged for a single induc- 
tion shock) or what is known as a supermaximal contraction— 
i.e, one in excess of what could be obtained by more gradual 
advances, which have no effect usually after a certain maxi- 
mum of contraction is reached. This, we think, a matter of 
considerable practical importance, and shall refer to its signifi- 
cance in a later chapter. 
Since the opening or closing of a key which makes or breaks 
the current really implies a very great change in the strength 
of the current affected suddenly—that is in fact from 0 to some 
+ quantity or the reverse—we find that usually the most marked 
contractions occur only at these times, and this holds, whether 
the current be slowly or rapidly made and broken (inter- 
rupted). 
The nerve being the natural means of conveying a stimu- 
lus, it is easy to understand how the contraction happens to 
follow most perfectly and with less strength of stimulus when 
this structure is excited. 
