36 THE VASCULAR SYSTEM OF THE FROG. 



than in frog's blood, are in the form of circular 

 biconcave discs with rounded edges, but no nuclei. 

 They have a tendency to run together into 

 rouleaux, like piles of coins. Their average 

 diameter is 0*008 mm., or about -g-oVo ^^ ^^ inch, 

 ii. "White corpuscles. These are very similar to those 

 of the frog : they are slightly larger than the 

 red corpuscles, averaging about 0"01 mm., or 

 "2 Wo" ^^ ^^^ ^^^^ ^^ diameter : their amoeboid 

 movements are not well seen unless the slide is 

 warmed. 



2. Action of acetic acid. 



Treat tvith acetic acid as before : note that, unlike the frog's 

 blood, no nuclei are visible in the red corpuscles. 



G. Circulation of the Blood in the Web of a Frog's Foot. 



The web uniting the toes of the frog's foot is so thin and 

 transparent, that with the microscope the blood in it can readily 

 be seen coursing along the capillaries. 



Examine a frog prepared to show the circulation in the web of 

 the foot. Note the following points : 



1. With a low power. 



a. The irregularly branched pigment cells to which the 



colour of the frog's skin is due. 



b. The fine meshwork of bloodvessels along wdiich the 



blood can be seen flowing. These bloodvessels are 



of three kinds. 



i. The arteries, carrying blood to the web, are dis- 

 tinguished by the fact that when they divide, 

 the direction of flow of the blood is from the 

 larger trunk to its branches. 



ii. The capillaries form a close network of very small, 

 very thin-walled vessels, along which the blood 

 flows from the arteries to the veins. 



iii. The veins, carrying the blood away from the web 

 back towards the heart, are distinguished from 

 the arteries by the fact that the blood in them 

 flows from smaller to larger vessels. 



