STIMULATION OF DIVIDED VAGUS NERVE. 301 



impossible to produce apncea after section of both vagi, and Knoll 

 stated that such was the case. As a matter of fact, however, although 

 difficult shortly after section of the vagi, it is not impossible to produce 

 an apnceic pause by ventilation with air. The ease with which this 

 effect is produced increases with the length of time that the vagi have 

 been divided. 



How is the production of apnoea under these conditions to be 

 explained ? Head ascribes its production to the fact that, after section 

 of the vagi, the aeration of the blood is not efficiently carried out, and 

 the blood gradually gets more and more venous. In consequence of its 

 lowered nutrition, the respiratory centre becomes adapted to this 

 dyspnoeic condition of the blood. If active ventilation be now carried 

 out, the blood, being normally aerated, is over-oxygenated with reference 

 to the condition of the centre, so that in this case the causation of the 

 apnoea is the same as that imagined by Eosenthal for the apnoea in 

 animals with intact vagi. 



As might be expected, it is impossible, after division of the vagi, to 

 cause apnoea by ventilation with hydrogen or nitrogen. The fact that it 

 is possible, even after section of the vagi, to produce an apnoeic pause by 

 means of artificial respiration, was first established by Miescher, 1 who 

 therefore distinguished apncea vera or chemical apncea, due to (relative) 

 over-oxygenation of the centre, from apncea vagi or mechanical apncea, 

 due to the inhibitory effects of repeated distension of the lungs. 



Where does the summation of the stimuli which cause the apnoeic 

 pause take place ? Gad, who was the first to explain the apnoea as due 

 to the summation of certain stimuli, believed that the summation 

 takes place in the peripheral end-organs of the vagi. Head has 

 shown, however, that if the vagi be divided at the beginning of the 

 apnoeic pause, this pause, instead of being cut off, is much more 

 pronounced than under normal conditions, showing conclusively that the 

 summation of stimuli must take place in the centre itself rather than in 

 the vagal terminations. 



Effects of artificial stimulation of the central end of the divided 

 vagus nerve. — Eosenthal pointed out that electrical stimulation (by 

 induction currents) of the central end of the vagus in the neck causes 

 an increase in the inspiratory rhythm, and with stronger currents an 

 actual tetanus of the inspiratory muscles. He ascribed all expiratory 

 effects to the escape of current into the superior laryngeal nerves. 

 Since then it has been shown by numerous observers that the constant 

 effect of the weakest current which can cause any change at all in the 

 respiratory movements is inhibitory. The pauses between the inspir- 

 atory contractions become longer than usual, and the inspirations are 

 generally less powerful. Exactly the same effect is produced when the 

 divided nerve is stimulated by its own demarcation current, or when a 

 constant current is passed through it in an ascending direction 

 (Langendorff and Oldag). 2 The statement of these observers, that on 

 opening the current, or on closing the current in a descending direction, 

 an inspiratory effect is produced, has been contested by later authors, 

 e.g. Lewandowsky. 3 With the ordinary induction currents this primary 

 expiratory effect is difficult to obtain with certainty, since a very slight 



1 Arch./. Physiol., Leipzig, 1885, S. 355. 



2 Arch./, d. c/cs. Physiol., Bonn, Bd. lviii. S. 201 ; cf. also Boruttau, ibid., Bd. Ixi. S. 39. 



3 Arch.f. Physiol., Leipzig, 1896, S. 195. 



