76 EXPERIMENTAL PHYSIOLOGY. 



conversion of an intermittent flow in the arteries to a continuous 

 flow in the veins. 



The apparatus consists of a reservoir R, a pump P, and 

 two lengths of tubing, one, A, of glass and the other, B, of 

 rubber. Each tube is provided at its extremity with a thumb- 

 screw by which its calibre can be varied. A pinch-cock, D, 

 placed between the reservoir and the pump, allows the flow of 

 fluid to be cut off when the schema is not in use. 



With the pinch-cock D closed, fill the reservoir with water. 

 Adjust the thumb-screws at the extremities of the tubes so 

 that the lumen of each is at its maximum calibre. Place a 

 basin beneath the openings of the tubes or direct them so that 

 the fluid issuing therefrom will flow into the sink. Open the 

 pinch-cock D. Compress the bulb intermittently at the rate of 

 about 60 per minute. What is the nature of the flow from 

 tubes A and B respectively? Next adjust the thumb-screws 

 so that the diameter of each tube is considerably reduced. 

 Repeat the intermittent compression of the bulb closing off 

 A or B by compressing the rubber tubing next the Y-piece. 

 What do you find with regard to the outflow from each tube? 

 Explain your results. 



Response of Blood Vessels to Chemical Substances. 



The amount of blood to any region can be increased by an in- 

 crease in the output of the heart or by a dilatation of the blood 

 vessels in that region. Dilatation may be brought about through 

 action of the nerves to the vessels or by chemical substances in 

 the blood. For example certain waste products such as lactic acid 

 may cause dilatation. The blood vessels therefore not only by 

 their elasticity help to maintain a steady blood flow, but by their 

 active constriction and dilatation shunt the blood to regions most 

 in need of it. That the blood vessels will respond to chemical 

 substances, can be shown by the following experiment. 

 Experiment 34. Perfusion of the Frog. Not only can the 

 heart be kept beating for a considerable time by perfusion, but 

 circulation can be maintained for some time through the blood 

 vessels as well. Arrange the beaker and siphon for perfusion 

 as used in the turtle heart. After exposing the heart of a frog 



