180 ANIMAL PHYSIOLOGY. 
the'screw, d, is in electric continuity with the wire, y, of the same primary coil. The screw, 
d, and the rod, ¢, are provided with platinum points, and both are insulated by means of 
the ebonite block, e. The circuit of the primary coil to which z and y belong is closed as 
‘long as c and d are in contact. When in its swing the tooth, a’, knocks c away from d, the 
circuit is immediately broken, and a ‘‘ breaking” shock is sent through the electrodes con- 
nected with the secondary coil of the machine, and so through the nerve. The lever, l, the 
end only of which is shown in the figure, is brought to bear on the glass plate, and when at 
rest describes an arc of a circle of large radius. The tuning-fork, f(ends only seen), serves 
to mark the time (after Foster). 
myograph (Fig. 177). It consists of a heavy pendulum, which 
swings from a position on the right to a corresponding one on 
the left, where it is secured by a catch. During the swing of 
the pendulum, which carries a smoked glass plate (by means 
of arrangements more minutely described below the figure), a 
tuning-fork writes its vibrations on the plate, on which is 
inscribed the marking indicating the exact moment of the 
breaking of an electric current, which gives rise to a muscle 
contraction that is also recorded on the plate. 
The tracing on analysis presents: 1. The record of a tuning- 
fork making one hundred and eighty vibrations in a second. 
2. The parallel marking of the lever attached to the muscle 
before it began to rise. 3. A curve, at first rising slowly, and 
then rapidly toa maximum. 4. A curve of descent similar in 
character, but somewhat more lengthened. 
We may interpret this record somewhat thus: 1. A rise of 
the lever answering to the shortening of the muscle to which it 
ab c 
Fic. 178.—Muscle-curve obtained by the pendulum myograph (Foster). Read from left to 
right. The latent period is indicated by the space between a and b, the length of which is 
measured by the waves of a tuning-fork, making one hundred and eighty double vibrations 
im a erie / and in like manner the duration of the other phases of the’contraction may 
e estimated. 
is attached following upon the momentary induction shock, as 
the entrance of the current into the nerve, the stimulation of 
which causes the contraction, may be called. 2. A period before 
the contraction begins, which, as shown by the time marking, 
occupies in this case 2k or about #, of asecond. In the tracing 
the upward curve indicates that the contraction is at first rela- 
