CIRCULA TION. 87 



oil. If no marked change is produced by the croton 

 oil, touch the point with a needle which has been 

 dipped into strong nitric acid. 



(7) Observe with a high power. Have you noted di- 

 apedesis of white or of red corpuscles. If so, describe 

 the process minutely. 



b. To observe the action of the frog's heart. 



/. Appliances. Dissecting board; fine scissors; heavy 

 scissors; pins; forceps; watch glass; camel's hair brush; 

 normal salt solution; fine silk thread; ice, in a beaker. 



2. Preparation. Pith a frog, lay it with its dorsal surface 

 upon the dissecting boaid; stretch out its legs and pin 

 the feet to the board. 



j. Operation Make a median incision through the skin 

 from the pelvis to the mandible; make transverse inci- 

 sions and pin out the flaps. Raise the tip of the epi- 

 sternum; insert a blade of the fine scissors under it and 

 divide it transversely, about ^ cm. anterior to the tip. 

 Raise the anterior segment of the sternum at the point 

 of the transverse incision; insert the blade of the strong 

 scissors under it and divide it longitudinally in the 

 median line. Withdraw from the board the pins which 

 fix the anterior extremities, make gentle, lateral traction 

 upon the fore feet until the split sternum is sufficiently 

 separated to afford a convenient working distance and 

 to plainly expose the whole heart. 



4? Observations. 



(1) Note rate of systole. 



(2) Note sequence of contraction of auricles, ventricle 

 and bulbus. 



(3) Note change in shape of different parts. 



(4) Note change in color and the position of the same 

 in the heart-cycle. 



