90 EXPERIMENTAL PHYSIOLOGY. 



inhibition, since it persists after both vagi are severed. What con- 

 clusions do you draw as to the cause of the fall? Finally stimulate 

 the peripheral ends of the depressor and sympathetic nerves. 

 The sympathetic stimulation sometimes quickens the heart. 



5. ESTIMATE THE CIRCULATION TIME FOR THE LESSER CIRCU- 

 LATION (Stewart's method). Ligate the carotid and remove the 

 cannul^. On the same side of the neck insert a cannula (pointing 

 towards the heart) in the jugular vein (for technique see par. 6, 

 Exp. 31). Expose the carotid artery of the opposite side and place 

 under it a strip of white glazed paper resting on a piece of thin 

 rubber dam. Throw a strong light on the artery. Connect the venous 

 cannula with a burette containing a 0.2% solution of methylene 

 blue in physiological saline solution, at the temperature of the 

 body. See that there is no air in the tubing. When all is ready, 

 remove the bull-dog forceps and mark the exact moment that the 

 methylene blue enters the vein (i.e., when the methylene blue is 

 seen through the cannula to enter the vein). Carefully observe the 

 carotid artery. The moment at which the blue appears in the artery 

 is also noted. Repeat several times, using always the same amount 

 of methylene blue. To obtain accurate results no methylene blue 

 solution should be allowed to escape on to the wound. The results 

 are to be given in relationship to (1) actual periods of time and 

 (2) the heart-beats. 



6. DETERMINE THE SEAT OF OXIDATION IN THE BODY. Inject 

 an excess of a stronger solution of methylene blue until the animal 

 is killed by the injections, and make a careful autopsy; examining 

 especially sections of the muscles, kidneys and liver, also the urine, 

 the blood in the mesenteric vessels, etc. The methylene blue will 

 be found to have stained the blood but not the tissues. The tissues 

 have reduced it to methylene white. This is known as Ehrlich's 

 experiment and it shows that the tissues are the seat of reduction 

 in the body. Note the effect which exposure to air has on the 

 colour of the cut organs. On standing exposed to the air the cut 

 tissues will become blue because of oxidation of the methylene 

 white to methylene blue. What conclusions do you draw from the 

 experiment? 



