EFFECTS OF DIVISION OF THE VAGI. 



297 



allowed to fall back into the wound, the effects of division of the 

 conducting tracts may be complicated by the occurrence of excitation in 

 consequence of closure of the demarcation current at the surface of the 

 cut nerve. In order, therefore, to observe the effects of pure division, 

 it is better to use the method adopted by Gad, 1 of freezing the nerve. Gad 

 has shown that, when the conductivity of an afferent nerve is abolished 

 in this way, no effective stimulation, so far as can be judged from the 

 reflexes, is produced, although, if a motor nerve be treated in the same 

 way, fibrillar twitchings of the muscle supplied by the nerve are 

 observed. In addition to this method, Head made use of another 

 devised by Biedermann, for extinguishing the conductivity' of the nerve. 

 The vagus is placed in a tube of indiarubber, so constructed that ether 

 vapour can be passed over it without affecting the atmosphere which 

 the animal breathes. In a few seconds the nerve is completely anaesthe- 

 tised and incap- 

 able of conducting 



impulses. On re- MMMMMMMMMfflMMfflMMMMMMMMMMi i> 

 moving the ether 

 vapour, by blow- 

 ing air through 

 the tube, the nerve 

 completely re- 

 covers its conduct- 

 ing power, and 

 this process can be 

 repeated several 

 times without any 

 apparent injury to 

 the nerve. 



Since the re- 

 sults of dividing 

 one vagus in this 

 manner are very 



transitory, and re- FIG. 172. Effects on respiratory movements of dividing (freezing) 



ciprnhlp in P Qmollav ^ rs ^ one > an( ^ then the other vagus, a, a, control curve, 



~? U "V* ' lclii showing passive movements of the chest wall ; b, b, curve taken 



degree those due by a tambour, showing changes in pressure in a bottle of air 



to division of both connected with the trachea ; c, c, myogram of diaphragmatic 



nerves we may slip ' ^' ^ me - markin g in seconds. Head, 

 pass at once to the effect of the latter proceeding. It is found invari- 

 ably that removal of the influence which the vagi normally exert on 

 the breathing leads to an increase in the force and duration of the in- 

 spiratory contractions, the first contraction which follows the division 

 far exceeding in extent both the previous and the subsequent con- 

 tractions. Division of the vagi affects the duration rather than the 

 height of each contraction of the diaphragm. The expiratory re- 

 laxations are as a rule incomplete, so that the base line of the 

 curve is raised, i.e. the diaphragm is in a state of tonic contrac- 

 tion (Fig. 172). In course of time the inspiratory tonus gradually 

 diminishes and the expiratory pause is more pronounced, so that 

 finally the breathing assumes the type which has already been described 

 as typical after section of both vagi. If the animal be deeply narcotised 

 or in a bad condition, or if there be any dyspnoea, the prolonged 



1 Arch.f. PhysioL, Leipzig, 1880, S. 1. 



