LABORATORY WORK 221 



If this regularity of compression is ensured, the effect of viscosity 

 may be investigated. Notice the height of the arterial pressure 

 when water is used. Then fill the system with 7 per cent, gum 

 arabic, without altering any of the adjustments. Repeating the 

 experiment, the pressure will be much higher. An instructive way 

 of doing the experiment is to disconnect between F and L, and put 

 the funnel into the end of the tube connected with K. Run in 

 gum solution through the funnel while the bulb A is being com- 

 pressed regularly. As the water is displaced the pressure in D will 

 steadily rise. 



Seven per cent, gum arabic has a viscosity about equal to that 

 of blood. It is easily made by placing the necessary amount in 

 water the day before it is wanted. The solution will need straining 

 through fine muslin or glass wool. To find out how much solution 

 is wanted, the system is filled with water and then emptied into a 

 graduated cylinder. 



The whole apparatus should be taken to pieces after use and 

 the parts kept separated. The sponges should be taken out of 

 their tube and dried. 



The Circulation in the Frog's Web 



Take a piece of thin wood, such as the lid of a cigar box, about 

 6 in. by 2 or 3 in. Near one end make a hole about half an inch 

 in diameter. 



Anaesthetise a frog by an injection of urethane. One minim of 

 a 5 per cent, solution for a medium-sized frog (25 gm.), under the 

 skin of the back, will be about the correct dose. When motionless, 

 lay the frog, belly downwards, on the board. Tie threads to two 

 of the toes, place a glass slide over the hole, and draw the web over 

 it by fixing the threads into notches in the end of the wood. 

 Do not stretch it tightly. Keep the frog moist by wet filter paper 

 on its back, and the web by occasionally dropping water on it. 

 Examine with low magnification, and note the network of blood 

 vessels below the layer of pigment cells. 



The arteries are recognised by the rapid current of blood in 

 them ; they run mainly towards the free edge of the web. The 

 direction of the flow is from larger to smaller vessels. 



The capillaries, in which the arterial branches end, form a close 

 network in which the current is slow. 



The veins are formed by union of capillaries. The blood-flow 

 is more rapid than in the latter, but not so fast as in the arteries. 

 The direction of flow is from smaller to larger vessels, and mainly 

 away from the free edge of the web. 



Next take a triangular bit of cover-glass, place it on the web 



