I'HYSinl.Oijy FOR DKNTAI, STl'DENTS. 



increase or decrease in the volume of the kidney will show an up and 

 down movement of the style. The pulse tracing obtained shows that 

 the instrument records even small changes in the kidney volume. If 

 the splanchnic nerve is stimulated, with the plethysmograph in place, 

 there is a great decrease in the size of the kidney as shown by the 

 fall in the writing style of the tambour. This is brought about by the 

 great vasoconstriction which accompanies the stimulation of the nerve 

 (see Fig. 26). 



DEMONSTRATION No. 3. 

 Factors which Influence Blood Pressure. 



A. The Effect of Afferent Stimuli on the Respiration and the Blood 



Pressure. 



The lingual branch of the fifth nerve is exposed on the under sur- 

 face of the jaw, and electrodes are placed on the central end (the end 

 towards the brain) of the divided nerve. While a normal blood pres- 

 sure tracing is being taken the nerve is excited by stimulation from 

 an induction coil with tetanizing shocks. A rise in blood pressure and 

 increased respiratory movements are observed with strong currents 

 in most cases. This is due to the afferent stimuli affecting the vaso- 

 motor and respiratory centers and reflexly influencing control of the 

 efferent respiratory and vascular nerves. 



B. The Effect of Stimulation of the Central End of the Cut Vagus 



Nerve. 



The vagus on one side is cut and the central end is stimulated witli 

 stimuli of varying strength. With very weak stimuli a fall in blood 

 pressure is usually produced. Stronger stimuli may produce a marked 

 rise in pressure. The effect is due to a reflex stimulation or inhibition 

 of the vagus and vasomotor centers. 



C. Effect of Haemorrhage on the Blood Pressure. 



A cannula is inserted into the femoral artery, and while a normal 

 blood pressure tracing is being made, the artery is opened. It will be 

 found that when the artery is fully opened, there is an immediate fall 

 in blood pressure, due to lessening of the peripheral resistance. If 

 the artery is only partially opened, considerable bleeding may occur 

 before the blood pressure is affected. The explanation for this lies in 

 the vasomotor center being stimulated by lack of blood and causing a 

 generally increasing vasoconstriction over the body. 



D. The Effect of Gravity on the Circulation. 



Through two staples on the under surface of the dog board and op- 



