INNEKVATION OF HEART AND BLOOD-VESSELS 569 



rhage. Expose the brain and the anterior half of 

 the spinal cord (page 565). Place the frog on the 

 web-board. Note carefully the speed with which 

 the corpuscles pass through the smaller vessels 

 of the web. The rate of flow in the capillaries is 

 the best practical index of the diameter of the 

 small arteries. When the arteries constrict, the 

 flow in the capillaries will be less rapid. Eemove 

 the cerebral hemispheres and the optic lobes. 

 After five minutes or more (to allow the frog to 

 recover from the shock of the operation), note the 

 condition of the web vessels. 



There will be no significant change. 



The removal of the brain anterior to the bulb 

 has not destroyed the ton us of the blood-vessels. 



Note the slow rhythmic changes in the diam- 

 eter of the vessels. The changes are not uniform 

 throughout the length of the blood-vessel. 



2. Curarize the frog sufficiently to paralyze 

 the motor nerves. Stimulate the bulb with very 

 weak tetanizing currents. 



The flow in the capillaries will be less rapid. 

 Obviously the bulb contains nerve cells, the ex- 

 citation of which causes the narrowing of the 

 blood-vessels. These cells are termed the bulbar 

 vasoconstrictor centre. Kepeated sections show 

 that the vasoconstrictor cells are placed (in the 

 rabbit) on both sides of the median line from 



