1192 
MONITORING 
Figure 3. — Caldwell type intramuscular electrode in hypodermic needle. The deinsulated portion of the tip is one 
millimeter. 
the tibialis anterior muscle of cats, and this 
implantation method will be described. A five 
centimeter incision is made over the belly of 
the tibialis anterior muscle and the subcutane- 
ous fascia separated by blunt dissection. The 
deep fascia of the muscle is then opened, ex- 
posing the muscle fibers. The hypodermic needle 
containing the Caldwell electrode is inserted 
into the desired portion of the muscle. The 
needle is slowly withdrawn while applying a 
slight pressure near the tip of the needle, leav- 
ing the electrode in the muscle. Connection of 
the small electrode to a more sturdy lead wire 
then follows. This is the lead wire which will 
eventually exit the animal at the interface. The 
lead wire is a helical coil wound from 130/x 
type 316 stainless steel wire (coiled diameter 
0.72 mm) and insulated with a silicone rubber 
tubing. (The insulated lead wire was obtained 
from Medtronic Inc., Minneapolis, Minnesota.) 
The insulation is pushed back from the tip of 
the wire to expose at least 15 mm of coiled 
wire. The coiled wire is then folded back on 
itself to form a U shape without permanently 
deforming the coil. At the outside portion of the 
bend, the coils are open and the free end of the 
Caldwell electrode can be passed down the cen- 
ter of the coiled lead to exit at the bend, shown 
in Figure 4. (The tool used in this figure to 
maintain the bend is formed by drilling two 
holes in the jaws of the needle holder.) Once 
the free end of the electrode exits the coiled 
lead wire at the bend, the lead wire can be al- 
lowed to straighten. Pulling the free end of 
the electrode will draw it into the lead wire. 
Pulling also strips the insulation from the elec- 
trode wire, allowing electrical contact between 
the lead wire and the electrode. Contact should 
be tested by applying an external stimulus and 
observing the twitch response. A drop of Silas- 
