65 



XXIII. 



295- 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 drop or two of a i % 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. 



296. If the frog is more convenient, prepare it by de- 

 .stroying 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 moi.st. 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. 



297. 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 di- 

 verging from the artery, while in the veins the corpuscles 

 accumulate from the capillaries converging to form the vein. 

 A slight pulsation may sometimes be observed in the smaller 

 arteries. 



298. Note the greater velocity of blood in the arteries than 

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

 made out in either. 



299. 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 leucocytes may be seen in the peripheral zone, if the 

 current is not too rapid ; in that of the veins a few leucocytes 



