182 ANIMAL PHYSIOLOGY. 
in which it is seen that the latent period in the latter case is 
shortened by the distance from b’ to 6, which must be owing 
to the time required for those molecular changes which, occur- 
ring in a nerve, give rise to a contraction in the muscle to which 
it belongs; in fact, we have in this method a means of estimat- 
ing the rate at which these changes pass along the nerve—in 
other words, we have a means of measuring the speed of the 
propagation of a nervous impulse. . The estimated rate is for the 
frog twenty-eight metres per second, and for man about thirty- 
three metres. As the latter has been estimated for the nerve, 
with its muscle in position in the living body, it must be re- 
garded rather as a close approximation than as exact as the 
other measurements referred to in this chapter. 
It will be borne in mind that the numbers given as repre- 
senting the relative duration of the events vary with the ani- 
mal, the kind of muscle, and a variety of conditions affecting 
the same animal. 
TETANIC CONTRACTION. 
It is well known that a weight may be held by the out- 
‘stretched arm with apparently perfect steadiness for a few 
seconds, but that presently the arm begins to tremble or vi-. 
brate, and soon the weight must be dropped. The arm. was 
maintained in its position by the joint contraction of several 
muscles, the action of which might be described (traced). by a 
writer attached to the hand and recording on a moving sur- 
face. Such a record would indicate roughly what had hap- 
pened ; but the exact nature of a muscular contraction in such 
a case can best be learned by laying bare a single muscle, say 
in the thigh of a frog, and arranging the experiment so that a 
graphic record shall be made. 
Using the apparatus previously described (Fig. 177), a second 
induction shock may be sent into the muscle before the effect 
Fia. 180.—Tracing of a double muscular contraction (Foster). A second induction shock was 
sent into muscle when it had so far completed its contraction as is indicated by beginning 
of second rise. Dotted line indicates what the curve would have been but for this. 
