vi PRACTICAL DIRECTIONS 103 



bulbus aorta, and the small aperture leading into the pulmo- 

 cutaneous trunks. 



4. The sinu-auricular aperture and its valves. 



5. The aperture in the left auricle leading into the pul- 

 monary veins. 



Sketch. 



Turn over the heart, so that its dorsal surface is upwards, 

 and cut away enough of the dorsal wall of the sinus venosus 

 to show the sinu-auricular aperture from the other side. 



V. Get a piece of thin board e.g., the side of a cigar-box 

 about six inches loftg 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 anesthetic as soon as 

 it is insensible, so that there may be no suspicion of the frog 

 feeling any inconvenience from this harmless experiment. 

 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 circula- 

 tion of the blood through the arteries, capillaries and veins 

 (Fig. 24). 



1 A brief description of the compound microscope will be 

 given at the end of the next chapter (p. 119). 



