VASOMOTOR REFLEXES 87 



distinguishable into three strands: the smallest of the strands will be 

 the depressor. The sympathetic in the undisturbed position of the 

 nerve stands lies medial to and slightly behind the vagus and is considerably 

 smaller than the vagus. 



The depressor nerve is a filament much smaller again than the 

 sympathetic and usually runs along with the vagus on the medial aspect 

 and slightly in front of that trunk : a minute blood-vessel on the vagus- 

 sheath often marks the separation between the vagus and the small nerve 

 sought. The sympathetic at this level lies usually on the medial side 

 of the vagus ; when it does so the small nerve — the depressor — lies between 

 them and along their superficial aspect. But sometimes the sympathetic 

 early crosses to the outer side of the vagus, and then the depressor does 

 not lie between them. If the depressor nerve is not made out by 

 inspection of the vagus and sympathetic before they are disturbed, it is 

 helpful to free vagus from the tissue it lies on and slip as lifter a small glass 

 slide under it, and r aise gently ; this flattens the trunk somewhat, and the 

 depressor twig running along with it can then be made out as a slender 

 strand separable from it, though more or less closely adherent to it. 

 If this search fails to find the depressor, search for it by turning to the 

 vago-sympathetic of the other side of the neck. 



With a fine-pointed needle mounted in holder scrape gently 

 through the tissue binding the depressor to the main vagus for a length 

 of a couple of millimetres at a level of at least 1 cm. below the cricoid 

 cartilage; then hgate the depressor at that point with fine silk. Cut it 

 distal, i. e. aboral, of the ligature. Lifting it by the hgature, carefully 

 isolate it from vagus for at least 2 cm. headward. 



Ligate and sever below the ligature the vagus trunk as low 

 down in the neck as practicable, and dissect the trunk up for at 

 least 2 cm. 



IV. Expose the carotid of the opposite side to that on which you Obs. 69. 

 have prepared the above nerves, and connect it with the kymograph Respiratory 

 manometer in the usual way (see exerc. V). Take a sample record of the Trtlrfar**"^ 

 arterial pressure. Note the natural respiratory undulation of the blood- pressure, 

 pressure, the rise foUotving inspiration, not preceding it as with pump- 

 ventilation (cf. exerc. V.) If the pressure level is irregular it can usually be 



made steadier by attaching the respiration pump-tube to the tracheal cannula 

 and artificially ventilating the preparation. 



V. Faradize the central end of the prepared depressor nerve 



