In degenerated nerve there is of course a total
loss of irritability, corresponding to the destruc-
tion of the axis cylinder.
Galvanotropism

Paramecium. — Connect two non-polarizable
electrodes through an open key with a dry cell.
On a glass microscope-slide make with normal
saline clay an enclosure about one centimetre
square and a few millimetres high. Place in
this a little hay infusion containing Paramecia.
Bring non-polarizable electrodes against two op-
posite sides of the clay cell. Examine the infu-
sion with a very low power. Close the key.
STIMULATION OF MUSCLE AND NERVE 99

Upon closure each Paramecium turns the an-
terior end of the body towards the cathode and
swims in that direction. In a very short time
the anodal region is free, and the Paramecia are
gathered at the cathode, where they remain so
long as the current flows.
Open the key.
The Paramecia now turn to the anode and
swim in that direction, but the anodal grouping
is less complete than the cathodal, and lasts but
a short time. Careful observation shows that
in Paramecium the galvanic reaction consists in
placing the long axis of the body in the current
lines. The outermost individuals in the liquid
will therefore describe a curve corresponding to
the curved outer current lines.
All protozoa and many other animals (for ex-
ample, the tadpole and the crayfish) show gal-
vanotropism, but in some, movement on closure
is toward the positive pole (positive galvano-
tropism).
These experiments on skeletal, smooth, and
cardiac muscle, on nerve, and on infusoria, sug-
gest that polar excitation occurs wherever a gal-
vanic current passes through irritable tissue.
