PRACTICAL EXERCISES 



215 



30. Section and Stimulation of the Cervical Sympathetic in the Rabbit. 

 Set up an induction-coil arranged for an interrupted current (Fig. 93, 

 p. 198), 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 of carbolic acid, the sponges and ligatures in cor- 

 rosive sublimate solution (o'i per cent.). Instead of sponges swabs cf^, 

 sterile gauze or cotton may be employed, and until the observations on 

 the nerve have been made it is better to use sterile o - 9 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 use 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. 61. 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, and take out the 

 ligature. Wash the wound thoroughly with corrosive sublimate, then 

 with sterile (boiled) water, and close it, the muscles being first brought 

 together by a row of interrupted sutures and then the skin by another 

 row. Since it is difficult to thoroughly disinfect the hair-follicles, and 

 a suture passed through a septic follicle is apt to give rise to suppura- 

 tion, subcutaneous stitches i.e., stitches passed by a curved needle 

 through the deep layer of the skin without coming through to the 

 surface may be employed. The wound is to be protected by a coating 

 of collodion. No other dressing is required. The animal is now 

 removed from the holder and put back to its hutch. The student must 

 examine it at least once a day for the next week, and study the differ- 

 ences between the two ears (p. 173) and the two pupils. 



31. Determination of the Circulation-Time. (a) Begin with an arti- 

 ficial scheme (Fig. 107). Fill the syringe with a o'2 per cent, solution 

 of methylene blue. Allow the water to flow from the bottle by loosen- 

 ing the clamp. Inject a definite quantity of the methylene-blue solu- 

 tion, and with a stop-watch observe how long it takes to pass from 

 the point of injection to the end of the glass tube filled with beads. 

 Make ten readings of this kind, and take the mean. Then raise the 



