BY DR. BURDON-SANDERSON. 



and the motor nerves of respiration (and particularly the 

 phrenic) is entirely different from that which exists between 

 the afferent and efferent nerve in the ordinary case of reflex 

 action. 3. Animal in the state of dysjmcea. When the blood, 

 instead of containing too much oxygen, contains too little, the 

 effect of excitation of the central end extends itself to all the 

 extra muscles which are at the time in action ; consequently, 

 the greater the dyspnoea, the greater is the number of muscles 

 which respond to the stimulus. This is best seen in an animal 

 in which after perforation of one side of the chest, respiration 

 is maintained artificially ; the same rabbit which has served 

 for the other experiments may be used. The result may be 

 varied according to the degree of dyspnoea produced, by regu- 

 lating the frequency and quantity of the injections of air. If, 

 for example, the dyspnoea is sufficient to bring into action the 

 external intercostals, intercartilaginei, and scaleni, all these 

 muscles contract simultaneously with the diaphragm when the 

 central end is excited, so that the chest remains during the 

 time that the key is open, in a state of tetanic expansion. 



103. Excitation of the Central End of one Vagus after Sec- 

 tion of both. By very careful graduation of the induced cur- 

 rent (with Helmholtz's modification), it is sometimes possible 

 to supply the precise degree of excitation to the vagus centre, 

 which is required to make up for the loss sustained by the sec- 

 tion of its afferent fibres, and in this way to restore the normal 

 respiratory rhythm. More frequently the experiment fails, 

 and effects are produced which correspond to those described 

 above. 



Exceptional Cases. It very frequently happens, particularly 

 in animals under the influence of chloral, that effects are pro- 

 duced by excitation of the central end which are just the oppo- 

 site of those which we regard as normal. The diaphragm, in- 

 stead of contracting, relaxes, and remains relaxed during the 

 whole time (see Fig. 255 1 ) that the key is open. The imme- 

 diate cause of this generally is that the nerve is exhausted. 

 The reason why it happens is that the vagus contains (in addi- 

 tion to the fibres which, when excited, act on the vagus centre 

 in such a way as to lessen the hypothetical resistance by which 

 it is normally prevented from discharging itself in muscular 

 contractions) other fibres, which in the language of physiolo- 



1 The tracing, Fig. 255, was obtained by the method described in 

 99. During the whole period of excitation the diaphragm remained 

 stationary in the position of ordinary expiration ; almost immediately 

 after, the rhythmical movements were resumed, the first movement be- 

 ing an ordinary inspiration. The period was interrupted by a single 

 respiratory movement, caused by the accidental removal of the elec- 

 trodes from the nerve. The notches in the horizontal part of the tracing 

 express cardiac pulsations. 

 21 



