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. Towards one end make a round hole about 

 half an inch in diameter, and opposite this, on either 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. The web may be 

 examined in the living animal without hurting it, or the animal 

 may be first chloroformed as directed on p. 31, but removed from 

 the influence of the anaesthetic as soon as it is insensible, when 

 the brain may be destroyed by the operation of 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 w r ood, 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, 1 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 tirculation 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. 



