and Laboratory Methods. 2G75 



convenient for this, as for many other uses in the laboratory. Fill the cell with 

 clear water, slip the frog's foot and the spreader into it and fasten the frog to 

 the board by two or three thumb-tacks through folds of the cloth, or with twine. 

 Use any objective having a sufficiently long working distance to reach the web. 

 To check the rate of flow of the blood apply gentle pressure on the frog's thigh with 

 a round pencil or penholder. To reverse the flow in the capillaries roll the pencil 

 toward the foot while continuing the pressure. 



B. Directions for demonstratijig the pulsation of the auricles and ventricles of 

 the heart of a fresh-water clam : 



On account of the great variation in the thickness and convexity of the shells 

 of different species it is best to select a specimen with thin valves and short lateral 

 measurement. Thin valves are more easily ground off and the flatter the valve 

 the thinner the cell required to hold the clam during projection. Having dis- 

 sected a specimen of the species to be used in projection and determined the 

 position of the heart and pericardial cavity with reference to the external mark- 

 ings on the valves, grind both of the valves thin — almost to the surface of the 

 mantle — over an area larger than the entire pericardial cavity. Use an ordinary 

 grindstone with plenty of water. Grind off too much rather than too little, but 

 do not destroy the shell along the hinge line. Place the clam in a dissecting 

 tray and cover with clear water. Break away and pick off the thin film of shell 

 remaining over the pericardial cavity. Strong forceps and small bone-cutters are 

 useful in this operation. Pick up the mantle lobe over the anterior end of the 

 pericardial cavity with fine forceps and cut it away with scissors. The pulsating 

 auricles are apt to get between the points of the scissors and be cut open so 

 that the blood escapes and the auricle collapses. When the shell and mantle 

 have been removed from both right and left sides of the pericardial cavity there 

 is seen to be an opening through the clam just in front of the auricles and below 

 the ventricle, and these organs expand into this space at each diastole. If the 

 surrounding tissues interfere with the view of the pulsating organ, threads may 

 be passed through the opening and brought around the shell and tied so as 

 to keep the pericardial space clear for the passage of light in projecting. The 

 specimen should now be kept entirely submerged in water during all subsequent 

 manipulations, or the delicate auricles, deprived of the support of the pericardial 

 liquid, may be destroyed. 



The cell to be used in mounting the clam for projection depends upon the 

 type of your projection apparatus. If it is fitted with a vertical reflector the 

 clam may be placed in a tray of water without any cover. This, at first thought, 

 may seem to be the easiest method, but it has this drawback that any vibration 

 of the apparatus causes waves which seriously interfere with the perfection of 

 the picture on the screen. The other method is to mount the clam in a large 

 cell with an open top. The lens used in projecting the pulsating heart is the 

 same as is used for lantern slides, unless the clam is a small one and the cell con- 

 taining it can be placed on the stage of the projection microscope and a low power 

 objective used. 



A very instructive addition to this demonstration consists in a study of the 

 effects of heat and cold on the rate of pulsation of the heart of this typical cold- 



