DEMONSTRATIONS. 141 



III. Influence of the Cardiac and Vasomotor Centres on the Circulation 

 and on the Motions of the Heart. 



The atlanto-occipital membrane having been previously exposed, the 

 carotid is connected with the kymograph. A record is taken, and the mean 

 arterial pressure measured. On faradization of the spinal cord, at the level of 

 the third vertebra, mixed effects are observed, due partly to the excitation of 

 the vascular nerves, partly to escape of induction currents to the cardiac centre. 

 If both vagi have been previously divided, those due to the latter cause do not 

 appear. The cord is now severed above the seat of excitation, respiration 

 being continued artificially : the arterial pressure sinks to a third of the previous 

 mean. The excitation is repeated ; the pressure rises rapidly, the heart beating 

 with great frequency. On opening the thoracic cavity, the action of the heart 

 may be studied. It is seen that so long as artificial respiration is continued, 

 it beats regularly. If the injections of air are intermitted for a few moments, its 

 cavities become more distended and its action more vigorous than before, and 

 a similar effect is produced by excitation of the spinal cord. 



IV. Functions of Vascular Nerves. 



Constricting Nerves. Division of the trunk of the sympathetic 

 opposite the cricoid cartilage is followed by dilatation of the central artery of 

 the lobe of the ear on the same side, and increase of vascularity. On compar- 

 ing the temperature of the congested lobe with that of the other side, it is found 

 to be two or three degrees higher. The pupil of the same side is more 

 contracted than the opposite one. Excitation of the end next the superior 

 ganglion produces constriction of the central artery and abolishes the conges- 

 tion of the lobe. 



Dilating Nerves. Excitation of the central end of the great auricular 

 nerve (or of the posterior auricular) produces temporary vascular changes, 

 which are identical with those permanently produced by section of the sympa- 

 thetic. 



Depressor Nerve. Excitation of the central end of the divided 

 depressor occasions general diminution of arterial pressure (dependent on 

 dilatation of the blood-vessels supplied by the splanchnic nerves). If the 

 vagi have been previously divided, the diminution of pressure is not asso- 

 ciated with any change in the frequency of the contractions of the heart. 



V. Movements of Circulation and Respiration in Man. 



i. The Cardiograph and Sphygmograph. a. Two receiving 



tympana (cardiographs) are used. One is applied to the seat of the cardiac 

 impulse, the other to the carotid artery. The two recording tympana with 

 which these are severally connected, inscribe the motion of the heart and that 

 of the artery respectively, on the same cylinder. The arterial expansion 



