THE FROG 



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 insert- 

 ing 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 destroying 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 preparing 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 with- 

 out shrinkage ; it preserves the central nervous system in a 

 very satisfactory condition for study. Formalin is pur- 

 chased 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 

 sufficient quantity or is changed once after two or three 

 days ; a stronger solution may make the specimens too rigid. 



After hardening the specimens for a few days or weeks 

 in formalin it is advisable to rinse them thoroughly in water 

 and change them to alcohol for permanent preservation, for 



