38 THE VASCtJLAft SYSTEM Of THE FfeOG 



ii. White corpuscles : become clearer, and show nuclei, 

 sometimes more than one in a single corpuscle. 



II. Human Blood. 



1. Normal. 



Prick the tip of your finger, and place a small drop of the 

 hlood on a slide : add a drop of normal salt solution, cover, and 

 examine as before. Note the following points : 



i. Red corpuscles. These, which are much smaller 

 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 rou- 

 leaux, like piles of coins. Their average diameter 

 is 0-008 mm., or about -g-^jjjg of an inch. 



ii. White corpuscles. These are very similar to those 

 of the frog : they are slightly larger than the 

 red corpuscles, averaging about O'Ol mm., or 

 2^Cio of an inch in diameter : their amoeboid 

 movements are not well seen unless the slide is 

 warmed. 



2. Action of acetic acid. 



IVeat with acetic add 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. 



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

 the foot. Note the follovring 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 which the 



blood can be seen flowing. These bloodvessels are 

 of three kinds. 



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



