VI PRACTICAL DIRECTIONS 103 



V. Get a piece of thin board — e.g., the side of a cigar box— about six 

 inches long by three wide. At the middle of one of the narrow sides 

 make a round hole about half an inch in diameter, and about half an 

 inch from each end of the same side make a notch or rather slit, with a 

 penknife. This is called a "frog-board." 



Next get as light coloured a frog as possible. Chloroform it as directed 

 on p. 31, but remove it from the influence of the ansesthetic as soon as 

 it is insensible, when the brain may be destroyed by the operation ot 

 pithing, so that there may be no suspicion of the frog feeling any 

 inconvenience from this harmless experiment. To pith the frog, feel 

 with the finger the joint between the skull and first vertebra on the 

 dorsal side, and with a sharp scalpel make a, small cut through the 

 skin and underlying tissue, so as to expose the spinal cord in this 

 region ; then rapidly insert a blunt instrument, such as a seeker 

 or a small piece of wood, into the cranial cavity, and move it 

 about until the brain is completely destroyed. Lay the frog on the 

 frog-board with a piece of wet rag wrapped loosely round the body, and 

 take one or two turns around both frog and board with a piece of tape — 

 you must avoid tying it tightly or the circulation will be impeded. 

 Stretch out one leg, and selecting the most transparent web, tie a piece 

 of thick soft silk round each of the two toes by which it is bounded. 

 Adjust the leg so that the web comes just over the hole in the frog- 

 board, and bring the two pieces of silk through the slits, regulating them 

 until the web is evenly stretched out over the hole. Lastly, place the 

 frog-board on the stage of the microscope,' with the hole over the 

 aperture in the stage, and either fix it with the clips or rest the opposite 

 end on some support : adjust the mirror so as to illuminate the web from 

 beneath, and examine it with the low power. Note the network of 

 capillaries and the circulation of the blood through the arteries, capil- 

 laries and veins (Fig. 24). 



1 A brief description of the compound microscope will be given at 

 the end of the next chapter. 



