298 PRACTICAL PHYSIOLOGY. [LX11. 



(e.) Begin pumping again until the mean arterial pressure is 

 restored, and then unclamp gradually the small arteries. The 

 steady fall of the blood-pressure represents the fall obtained when 

 the central end of the depressor nerve is stimulated (the vagi 

 being divided). 



(/.) Two sphygmographs may be adjusted on the arterial tube, 

 one near the heart and the other near the capillaries, tracings 

 being taken and compared. 



ADDITIONAL EXERCISES. 



4. Rigid and Elastic Tubes. Arrange an experiment as shown in fig. 221. 

 The flask should at least hold a litre, and be arranged as a Marriotte's flask. 

 The tubes one of glass and the other of caoutchouc have the same diameter, 

 and the outflow orifices are of the same size. The glass tube is attached by^a 

 short elastic tube to the lead tube coming from the reservoir. As the fluid 



FIG. 221. Marey's Scheme for showing that in the Case of Rigid and Elastic Tubes of the 

 same Calibre, under certain Conditions, the Elastic Tube delivers more Fluid than 

 the Rigid one. 



flows into the tubes, they are compressed rhythmically to imitate the inter- 

 rupted beat of the heart. Observe that more fluid is discharged by the elastic 

 than by the rigid tube. 



5. The Rheometer (fig. 222) is used to measure the amount of blood 

 flowing through a vessel in a given time, and, therefore, the diameter of the 

 vessel being known, to estimate the velocity or rate of blood-flow through an 

 artery. The nozzles of the instrument are inserted and tied into the artery 

 of an animal, but as the student is not permitted to do this, use an india- 

 rubber tube to represent the artery. 



