126 PRACTICAL PHYSIOLOGY 



On exciting the peripheral end of the nerve with the faradic current, 

 the vessels in the ear will be seen to constrict, and this will take place 

 to such a degree that all the smaller vessels will disappear from view. 

 The ear will at the same time become several degrees cooler. Note 

 that the latent time is considerable between the excitation and the 

 effect. Note that the pupil also dilates, the nictitating membrane 

 retracts, and the palpebral fissure is widened. The eyeball at the same 

 time projects forwards. The pupillo-dilator fibres arise from the first 

 three thoracic anterior roots, the vaso-con stricter fibres from the second 

 to the fifth, and even to the seventh, in the rabbit. If the superior 

 cervical sympathetic ganglion be painted with nicotine, excitation of 

 the preganglionic^ fibres will no longer have any effect on the ear, while 

 excitation of the post ganglionic fibres will still be effectual. The 

 sympathetic fibres to the head have their cell-stations in this ganglion. 



The Circulation Time of the lesser Circulation. The carotid artery 

 is exposed in the anaesthetised rabbit. A piece of thin rubber membrane 

 is placed beneath it. Between the membrane and the artery a piece of 

 white paper is inserted. The artery is illuminated by a lamp and 

 condensing lens. 



The external jugular vein is exposed on the other side of the neck, 

 and into its central end a cannula is inserted. A burette containing 

 methylene blue (sat. sol. in normal saline) is attached to the cannula, 

 a clip being interposed. No air must be left in the connections. With 

 a stop-watch, or by means of an electric signal and drum, note the time 

 which elapses between the injection of 1-2 c.c. of the solution of 

 methylene blue and its appearance in the carotid artery. 



CHAPTER XXXIV. 

 BLOOD PRESSURE. 



Demonstration of Arterial and Venous Pressure by the Method of 

 Stephen Hales. An incision is made in the mid-line of the neck, from 

 the larynx to the sternum of the anaesthetised animal. The skin-flaps 

 are pulled apart, and the sterno-mastoid and sterno-thyroid muscles 

 separated, so as to expose the carotid artery. With an aneurism needle 

 the artery is freed from the carotid sheath for the space of about an 

 inch. Two ligatures are then placed beneath the artery, and one is 

 tied at the upper end of the exposed portion. On the lower end an 

 artery clip is placed. With sharp scissors an oblique cut is next 

 made into the artery, and the nozzle of the arterial cannula is 



