THE FROG 63 
killed by drowning or asphyxiation in an air-tight jar, but this 
requires considerable time. 
In case the frogs are to be preserved immediately after anzs- 
thetizing, an incision should be made through the skin and ab- 
dominal wall, a little to one side of the median line, to admit the 
preserving fluid to the body cavity. 
In special cases it may be advantageous to kill the frog by an 
operation known as “ pithing.’”’ With the finger-nail locate a 
notch in the skeleton of the frog on the dorsal side between the 
skull and the first vertebra. Through this notch make a deep 
incision with a knife, cutting across the central nervous system. 
Death is probably instantaneous, but for certain purposes it is 
desirable to destroy the brain by inserting a needle or a seeker 
through the incision into the cranial cavity and stirring up the 
contents. The movements of the limbs of the frog may be 
quieted by inserting the seeker in the opposite direction and de- 
stroying the spinal cord. The method has the advantage of 
rapidity in case only a few specimens are to be killed, and, if 
the spinal cord is not destroyed, is especially useful for reflex 
action experiments; of course it should not be used if the central 
nervous system is to be dissected. 
To preserve the frogs they should always be placed for a time 
in formalin, which has many advantages over alcohol in prepar- 
ing material for anatomical work. Formalin does not extract the 
colors so rapidly as alcohol; it causes a slight swelling of some of 
the tissues, keeping the specimens in a plump condition, and in 
particular hardens the muscles without shrinkage ; it preserves the 
central nervous system in a very satisfactory condition for study. 
Formalin is purchased in a concentrated solution containing 40 per 
cent. formaldehyde; for practical purposes this is regarded as 
100 per cent. formalin. A 2 per cent. solution is strong enough 
for preserving the frogs, providing the solution is used in suff- 
cient quantity or is changed once after two or three days; a 
stronger solution may make the specimens too rigid. 
A‘fter hardening the specimens for a few days or weeks in 
formalin it is advisable to change them to alcohol for permanent 
preservation, for the following reasons: (a) formalin specimens 
are unpleasant to handle during dissection since the formalin 
