XVIl] BLOOD VESSELS. CIRCULATION 137 



muscular coat will also show as black transverse lines, these are 

 more numerous in the arteries than in the veins. 



6. Circulation of Blood 1 . The frog given you 

 has been deprived of its brain and ctirarised (1 drop 

 1 p.c. curari). Lay it on its belly on the frog-board, 

 and tie, not too tightly, a piece of soft cotton round 

 the end of the (e.g.) 3rd and 4th digits. Stick two 

 pins into the board a little distance from the hole ; 

 by twining the cotton round them the web may be 

 stretched out level above the hole (or the toes may 

 simply be pinned out with hedgehog quills, and the 

 quills cut short). Surround the web, and cover the 

 leg and body of the frog with moist blotting-paper. 

 Put a piece of cover-slip over the toes, and with a 

 small brush press the web from below against the slip. 

 (If the web is not in close contact with the glass it 

 may be outside the focal distance of the high objective; 

 in that case, a triangular piece of cover-slip should be 

 placed on the web between the toes, and excess of 

 fluid sopped up, but by this method the front lens 

 of the objective is apt to be smeared by touching the 

 tissue.) 



Examine, first with a low and then with a high 

 power. Note 



The course of the blood from the arteries to the 

 veins. A slight pulsation may be observed in the 

 larger arteries and sometimes in the smaller ones. 



The greater velocity of the blood in the arteries 



1 In the spring it is best to take a male frog for the circulation in 

 the mesentery ( 9). The male may be recognised by the wart-like 

 projection on the ball of the thumb. 



