320 RESPIRATION. 



opened. The effect cannot be predicted with certaintj". 

 Probably the respiratory movements will be quickened, the 

 lever assuming a somewhat higher position during the period 

 of excitation than it did before. This indicates that the dia- 

 phragm descends further in each inspiration, and does not 

 relax quite so much in expiration. 



The secondary coil must now be gradually brought up 

 nearer, while the excitation is repeated after each shifting, 

 until it is observed that the lever ascends and remains station- 

 ary each time the key is opened, drawing a nearly horizontal 

 line at a much higher level than that of the previous part of 

 the tracing. (See fig. 254 6. 1 ) If the excitation is continued 

 only for a few seconds, the elevation of the lever which indi- 

 cates contraction of the diaphragm not only continues during 

 the whole time, but lasts a second or two after it. The lever 

 then gradually falls, and after a few moments resumes its up- 

 and-down movements, always beginning with a descent. In 

 other words, the diaphragm, after a period of contraction, 

 which somewhat exceeds its cause in duration, is for a moment 

 relaxed before it assumes its rhythmical action. The conduct 

 of the other respiratory muscles should be carefully watched 

 (by another observer) during these experiments. It will be 

 seen that, provided that the animal is breathing perfectly 

 naturally at the moment that the key is opened, the descent 

 of the diaphragm determined by the excitation of the vagus 

 is not attended by any other muscular movement, and in par- 

 ticular, that the upper ribs remain as motionless as before, 

 and that the larynx does not descend. 2 2. Animal in the 

 state of apnosa. In a rabbit of which the blood has been sur- 

 charged with oxygen by excessive artificial respiration, the 

 effect of exciting the central end of the vagus is negative. 

 No respiratory movement is produced. To demonstrate this, 

 experiments must be made before, during, and after apnoea. 

 It is found that the same current which tetanizes the dia- 

 phragm in the normal state, has no effect when the blood is 

 over-arterialized. This is an experiment of fundamental im- 

 portance, because it shows that the relation between the vagus 



1 Fig;. 2545 shows that during the whole period of excitation (indi- 

 cated by the horizontal line below) the diaphragm remained contracted ; 

 then followed a few irregular movements, after which the rhythmical 

 movements were resumed with a slightly increased frequency. The 

 period of contraction was interrupted, as frequently happens, by a 

 momentary relaxation. 



2 Fig. 254a was obtained in the same animal as 2546 with the aid of 

 the apparatus, Fig. 250. The tracing shows that the rhythmical move- 

 ments were not resumed until a second or two after excitation had 

 censed. They were at first somewhat more frequent than before, and 

 the diaphragm was in a lower position. In less than a minute the pre- 

 vious conditions were restored in both respects. 



