BY DR. BURDON-SANDERSON. 319 



The phenomena which accompany excitation of the central 

 end of the divided vagus vary according to the state of the 

 animal and the state of the nerve. It will be convenient to 

 describe them under heads corresponding to these conditions : 

 1. Animal breathing naturally. To observe what may be re- 

 garded as the normal results of excitation, care must be taken 

 that the subject of experiment is not exhausted, and that, in 

 placing it on the support, nothing is done which can interfere 

 with its breathing. The movements of the diaphragm must 

 be recorded 1 either with the aid of the apparatus, fig. 250, or 

 in the manner described in the preceding paragraph ; but for 

 the present purpose, by far the best method is to introduce 

 into the peritonseal cavity, by means of a small opening in the 

 linea alba close to the ensiform cartilage, a small flat bag of 

 India-rubber, of such size that it can be conveniently slipped 

 between the diaphragm and liver. If this bag is slightty dis- 

 tended with air and connected with a Marey's tympanum, it 

 gives excellent tracings of the diaphragmatic movements. To 

 the student who witnesses the experiment for the first time, a 

 still more convincing mode of appreciating the effect of ex- 

 citing the central end on the diaphragm is to feel the con- 

 traction of the muscle with the finger during the period of ex- 

 citation. The nerve having been prepared, and the excitor 

 placed under it, a preliminary tracing must be taken of the 

 normal respiration. In a tracing, taken by the method de- 

 scribed in 99, it is seen that in each respiratory act three parts 

 may be distinguished, one of which, the ascent, expresses in- 

 spiration, or active contraction of the diaphragm ; the whole 

 of the remainder of the period corresponds to relaxation 01 

 that muscle. Sometimes the part of the curve which imme- 

 diately precedes the ascent indicates that towards the close of 

 the period of relaxation air neither enters nor leaves the chest. 

 If a straight line is drawn through the angle formed in each 

 curve at the point corresponding to the commencement of in- 

 spiration, it may be taken as indicating the position of the 

 lever when the diaphragm is at rest after an ordinary expi- 

 ration. So long as air is passing out of the chest, the lever 

 keeps below this line, but as soon as the outflow ceases, pro- 

 vided that the diaphragm is still relaxed, it returns to it. 

 Hence the line corresponds to the position of equilibrium. 

 These facts are well seen in the first (normal) part of tracing, 

 fig. 255. 



The tympanum having now been connected with the bag 

 between the diaphragm and liver, as above described, and the 

 secondary coil placed at a considerable distance from the pri- 

 mary, the key which has been connected with the telegraph is 



1 See 62:. 254a. 



