73 



276. If the caudal lymph-hearts are still beating-, pass a 

 tracer or piece of wire down the spinal canal to destroy the 

 myel. If thoroughly destroyed the lymph-hearts will cease 

 to beat. 



XIX 



277. The circulation of blood. This may be shown very 

 nicely in the delicate external gill filaments of the Necturus, 

 or in the tail of a tadpole, or in the web of a frog's foot which 

 does not contain too much pigment. The animals should be 

 injected with a few drops of a 1% solution of curare, in order 

 that they may not move, and arranged upon the stage of the 

 microscope, so that the parts to be examined may come clearly 

 into the field of vision. Precautions should be taken against 

 drying, by keeping the animal well surrounded with moist 

 cloth or absorbent cotton. 



278. If the frog is more convenient, prepare it by destroy- 

 ing the brain and injecting the curare under the skin of the 

 back. Place the frog on its belly on the frog board and pin 

 out the digits so that the web will be slightly on the stretch. 

 Keep the parts moist. Put a very small drop of water upon 

 the web, and cover it with a triangular piece of cover-glass, 

 being careful that it does not cut the digits and that no fluid 

 flows over its surface. Examine first with a low power, and 

 then, if possible, with a high power. 



279. Note the course of the blood from the arteries to the 

 veins. Arteries may be distinguished from veins by the fact 

 that the blood corpuscles scatter to enter the capillaries diverg- 

 ing from the artery, while in the veins the corpuscles accumu- 

 late from the capillaries converging to form the vein. A 

 slight pulsation may sometimes be observed in the smaller 

 arteries. 



280. Note the greater velocity of blood in the arteries 

 than in the veins; the individual corpuscles cannot, perhaps, 

 be made out in either. 



281. Note the axial and peripheral zones in the arteries 

 and veins ; the peripheral zone is small and under a low power 

 appears free from corpuscles ; under a high power a few leuco- 



