202 A MANUAL OF PHYSIOLOGY 



pressure rises* owing to constriction of the arterioles by direct 

 excitation of the junction between their vaso-constrictor nerves 

 and their muscular tissue. The heart is slowed, but its beat is 

 strengthened. At once cut both vagi while a tracing is being taken ; 

 the blood-pressure rises still more (p. 156). The rise of pressure is 

 sometimes so great that to prevent the mercury from being forced 

 out of the manometer the tube must be clipped. The rise is not long 

 maintained, but a second injection causes a renewed increase of 

 pressure. 



28. Section and Stimulation of the Cervical Sympathetic in the 

 Rabbit. Set up an induction-coil arranged for an interrupted 

 current (Fig. 81, p. 184), and connect it through a short-circuiting 

 key with electrodes. The preparations necessary for an operation 

 with antiseptic precautions are supposed to have been previously 

 made the instruments, sponges, and ligatures boiled in water ; 

 the instruments then immersed in a 5 per cent, solution ojf carbolic 

 acid, the sponges and ligatures in corrosive sublimate solution (o - 1 per 

 cent.). Instead of sponges swabs of sterile gauze or cotton may be 

 used, and until the observations on the nerve have been made it is 

 better to use sterile o'g per cent, salt solution for such slight sponging 

 as the wound may require rather than the antiseptic solutions. The 

 hands are to be thoroughly washed, with diligent US3 of the nail- 

 brush, in soap and water before the cutting operation begins, and then 

 soaked successively in alcohol and in the corrosive sublimate solution . 



Fasten the rabbit on a holder, back downwards, as in Fig. 51. 

 Keep the animal warm by covering it with a cloth, and do not handle 

 or wet its ears. Clip off the hair on the anterior surface of the neck. 

 Remove loose hairs with a wet sponge, shave the neck, and wash it 

 thoroughly, first with soap and water and then with corrosive 

 sublimate. Give ether. Make a longitudinal incision in the 

 middle line over the trachea, beginning a little below the thyroid 

 cartilage and extending downwards for an inch and a half. Feel for 

 the carotid artery, expose, and raise it up. Two nerves will now be 

 seen coursing beside the artery. The larger is the vagus, the smaller 

 the sympathetic. A third and much finer nerve (the depressor, or 

 superior cardiac branch of the vagus) may also be seen in the same 

 position, but the student should neglect this for the present. Pass 

 a ligature under the sympathetic, and tie it, the ear being held up to 

 the light while this is being done, so that its vessels may be clearly 

 seen. A transient constriction of the arteries may be seen at the 

 moment when the nerve is ligatured. This is due to stimulation 

 of the vaso-constrictor fibres. Then follows a marked dilatation 

 of the bloodvessels, due to paralysis of these fibres. The ear is 

 flushed and hot. Note also that the pupil is probably narrower on 

 the side on which the nerve has been tied. On stimulation of the 

 upper (cephalic) end of the sympathetic with the electrodes, the 

 vessels are markedly constricted, the ear becomes pale and cold, 

 and the pupil dilates. Cut the nerve above and below the ligature 



* The amount of the initial rise of pressure is very variable, since the 

 slowing of the heart tends to diminish the pressure, while the constriction 

 of the arterioles tends to increase it. Thus, in one experiment the increase 

 of pressure on injection of the extract was only 6 mm. of mercury, while 

 in another it was 56 mm. On section of the vagi in this second experi- 

 ment, there was an additional rise of 64 mm., and after a second injection 

 a further rise of 70 mm., making an increase of 190 mm. in all above the 

 original pressure. 



