;98 A MANUAL OF PHYSIOLOGY 



inhibition of the heart or acceleration, and the pressure may fall 

 or rise (p. 154). 



(c) Expose and divide the other vago-sympathetic while a tracing 

 is being taken. Again stimulate the central end of the nerve and 

 observe whether there is any effect. 



(d) Expose the sciatic nerve in one leg, as follows : The leg having 

 been loosened from the holder, the foot is seized by one hand and 

 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. 

 Dilatation of the pupil is also caused by stimulation of the upper 

 end of the vago-sympathetic through the sympathetic (pupillo-dilator) 

 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 how long 

 it is possible to keep the heart from beating. Sometimes, but 

 rarely, 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 enter the 

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

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

 when the circulation has stopped completely and the pressure has 

 become equalized throughout the whole vascular system, a residual 

 pressure of only a few mm. (usually about 10 mm. Hg) is indicated. 

 In order to get the true zero pressure, disconnect the arterial can- 

 nula from the manometer, and allow the writing-point to trace a 

 horizontal straight line (line of zero pressure) on the drum (Figs. 72 

 and 73). 



23. Estimation of the Arterial Blood-pressure in Man. With the 

 Erlanger sphygmomanometer estimate the systolic and diastolic 

 pressures in the brachial artery of a fellow-student as described on 

 p. 105. Begin with the observed person in the sitting position. 



Compare the results with those obtained on the same artery with 

 any other sphygmomanometer which may be available, especially 

 with one like the Riva-Rocci, with which the systolic pressure is 

 obtained by observing the height of the mercurial manometer at 

 the moment when the pressure in the cuff over the brachial has 

 fallen to the point at which the pulse at the wrist is just obliterated. 

 By compressing rapidly the rubber bulb the mercury is first raised 

 to a height somewhat greater than that necessary to completely 

 obliterate the radial pulse. Then the bulb is kept compressed, and 

 the mercury allowed to fall steadily, the point being noted at which 

 the fingers over the radial just perceive the returning pulse. The 

 observer's left hand may be used for palpating the pulse, and the 

 right for working the bulb. Repeat the observations with the 

 person standing up and lying down. Investigate the effect of 

 muscular exercise on the blood-pressure. 



