CIRCULATION OF THE BLOOD. 



A good index of general vasomotor changes is the speed of 

 blood-flow through the capillary vessels of the frog's web. If the 

 arterioles supplying the capillaries are constricted, the speed of 

 flow through the capillaries will be diminished. If the arterioles 

 are dilated, the speed of flow through the capillaries will be in- 

 creased. 



Keeping the brain and cord moistened with physiological salt 

 solution, arrange the web under a microscope for observation with 

 a medium high power. Note the size of the larger vessels, the ar- 

 terioles and venules, and the speed of flow through the capillaries. 

 The diameter of one of the arterioles may be measured by means 

 of the micrometer eyepiece. 



Excise the cerebral hemispheres and optic lobes, controlling 

 hemorrhage, if necessary, by packing. Allow five or ten minutes for 

 the frog to recover from the shock of the operation. Now examine 

 the web again. Is there any observable change in the size of the 

 arterioles or in the speed of blood-flow through the capillaries? 

 Has there been any marked interference with the tone of the blood- 

 vessels? What conclusion can you draw concerning the presence 

 of a vasomotor centre in the parts excised ? 



Observe the web for some time. Are there any rhythmical 

 changes in the diameters of the larger vessels discernible ? 



2. Inject a drop of saturated curare solution under the skin of 

 the frog in order just to paralyze the motor nerves without affect- 

 ing the innervation of the blood-vessels. This is done to keep the 

 frog quiet during the ensuing stimulations. Set up an inductorium 

 arranged for weak tetanizing currents. 



While observing the flow through the web, stimulate the medulla 

 with fine needle electrodes. What is the effect on the blood-flow 

 through the web ? Is this a constrictor or dilator effect ? 



3. Sever the medulla from the cord by a clean transverse cut 

 with a sharp scalpel. Again observe the flow through the web. 

 Is the speed increased or decreased? Is there a constriction or 

 dilatation of the arterioles? What is the function of the centre in 

 the medulla ? Is its action constant or intermittent ? 



[mi 



