i86 



A MANUAL OF PHYSIOLOGY 



lifted straight up, so as to put the skin of the thigh on the stretch. 

 An incision is now made in the middle line on the posterior aspect 

 of the thigh, through the skin and subcutaneous tissue. The muscles 

 are separated in the line of the incision with the fingers, and the sciatic 

 nerve comes into view lying deeply between them. Place a double 

 ligature on it, and divide between the ligatures. Stimulate the upper 

 (central end) ; the blood-pressure probably rises, and the heart may 

 be accelerated. Stimulate the peripheral end of the nerve ; there is 

 little change in the blood-pressure and none in the rate of the heart. 

 (e) Note, incidentally, that stimulation of the central end of the 

 sciatic or the upper (cephalic) end of the vago- sympathetic may 

 cause increase in the rate and depth of the respiratory movements. 



FIG. 73. BLOOD-PRESSURE TRACING FROM A DOG. STIMULATION OP 



CENTRAL AND PERIPHERAL ENDS OF VAGUS. 



The other vagus was intact. Stimulation of the peripheral end caused stoppage ol 

 the heart and a marked fall of pressure. Stimulation of the central end produced a 

 great rise of pressure, with, perhaps, a slight acceleration of the heart. 



Dilatation of the pupil may also be caused by stimulation of the 

 upper end of the vago-sympathetic through the sympathetic fibres 

 that supply the iris. 



(/) Again stimulate the peripheral end of one vagus, or of both 

 at the same time, while a tracing is being taken, and see ho ? long it 

 is possible to keep the heart from beating. Sometimes in the dog 

 inhibition can be kept up so long that the animal dies. 



(g) Close the tracheal cannula so that air can no longer mter the 

 lungs. In a very short time the blood-pressure curve begins to rise 

 (rise of asphyxia). After some minutes the pressure falls, ar d finally 

 becomes zero ; i.e., the level of the mercury is the same in the two 

 limbs of the manometer (or, rather, the mercury in the distal limb is 



