BY DR. BURDON-SANDERSON. 815 



trial). At the close of the experiment, the liquids are trans- 

 ferred once more to flasks, similar to those above described, and 

 their strength determined as before. The calculation of the 

 result is simple. The quantity of carbonic acid absorbed by 

 each 30 cub. cent, of the liquid is indicated by the difference 

 between the corresponding quantities of oxalic acid solution 

 used, before and after absorption. This quantity must be mul- 

 tiplied in the one case by 1 5 3 IJ 5 =4.5,in the other by =3- The 

 sum of the two products is the total quantity of carbonic acid 

 disengaged during the period of observation. If an animal 

 larger than a guineapig is used, it is necessary to employ two 

 sets of absorbing tubes, or a greater number. 



SECTION V. INNERVATION OF THE RESPIRATORY MOVEMENTS. 



The rhythmical movements of respiration depend on the 

 activit}' of a centre contained in that part of the floor of the 

 fourth ventricle from which the roots of the vagus nerve 

 spring. The proof of this fact lies in the fundamental ex- 

 periment of Legallois, by which he showed that the cerebrum, 

 the cerebellum, and even part of the medulla oblongata itself 

 may be removed, without arresting respiration. This experi- 

 ment has already been described in 92. 



B}' motor nerves this centre is in relation with the muscles 

 of which the combined rhythmical actions have been studied in 

 the same paragraph. Its discharges of energ}', like those of 

 the motor centres of the heart, are automatic, but their rhythm 

 is constantly subject to modification by impressions received 

 through the afferent fibres of the vagi. Consequently, the study 

 of the innervation of the respiratory movements resolves itself 

 into experiments relating to the respiratory functions of these 

 nerves. The results of such experiments may be divided accord- 

 ing as they relate to the effects of section of both vagi, to ex- 

 citation of the central end of the divided nerves, or to excita- 

 tion of the superior laryngeal nerve. 



99. Section of both Vagi in the Neck. In the para- 

 graph relating to the functions of the vagus as a heart-nerve, 

 directions have been given as to the mode of preparing it. The 

 rabbit is preferable to the dog or cat, for in those animals the 

 vagus is united in one trunk with the sympathetic. Section 

 of the vagi is the simplest and at the same time one of the most 

 instructive experiments relating to the physiolog}' of the ner- 

 vous system. The animal having been secured in the usual 

 way on Czermak's rabbit support, a ligature is passed round 

 each nerve a little below the cricoid cartilage. The ends of 

 each ligature are then knotted together, so as to facilitate their 

 being found at any moment. To observe the effect, the animal 

 should be placed before and after section, under the same cir- 



