296 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



Subsequent investigations, while confirming the account given by 

 Hering and Breuer of their experiments, have introduced certain modi- 

 fications into their interpretation of them. It is curious to note that, 

 whereas Eosenthal regarded the lung fibres of the vagus as purely in- 

 spiratory in function, the researches since the publication of Hering and 

 Breuer's paper have tended more and more to exalt the expiratory func- 

 tions of these fibres, some authors going so far as to deny the existence in 

 the vagus of any fibres whose function it normally is to promote inspiration. 



Of these investigations, the most important are those carried out by 

 H. Head 1 in Hering's laboratory, since they not only embrace a number 

 of experiments of the most varied kind on the functions of the vagi, but 

 they also represent the only series of researches in which the method of 

 experiment was free from objection and really adapted to the object in 

 view. In order to record the respiratory movements, all other investi- 

 gators have used the depression of the diaphragm, the elevation of the 

 ribs, or the movements of the air in the lungs. These methods are 

 obviously inapplicable when we wish to study the normal mode of 

 stimulation, i.e. distension or collapse of the lungs, since these events 

 cause a passive movement of the recording lever which it is difficult to 

 dissociate from the active movement caused by the contraction of the 

 inspiratory muscles. In Head's method, as has been already described, 

 the activity of the respiratory centre is gauged by recording the contrac- 

 tions of an isolated slip of the diaphragm, which is practically unaffected 

 by passive movements of the chest wall. In the succeeding account the 

 results of Head's experiments will be mainly followed. 



1. The normal respiratory movements. Each inspiratory contrac- 

 tion of the diaphragm, as sampled by the isolated slip, begins rapidly, 

 but soon becomes slower, though the lever is still rising when the con- 

 traction suddenly ceases. The first part of the expiratory relaxation is 

 extremely rapid, but the fall of the lever generally gets slower towards 

 the end of the relaxation. The writing point then traces a horizontal line, 

 representing the complete elongation of the muscular slip (Fig. 168). 

 In the rabbit the expiration is in most cases entirely passive. Where an 

 active expiratory movement takes place, it begins gradually towards the 

 end of the expiratory pause, and increases until it is suddenly broken in 

 upon by the rapid inspiratory contraction of the diaphragm, which is 

 accompanied by a complete relaxation of the expiratory muscles. Very 

 often, when the breathing is rapid, the respiratory pause may be entirely 

 wanting, and in such cases it is often found that the lever does not 

 quite descend to the base line between each inspiratory contraction, 

 so that each contraction starts, as it were, from the summit of a tonic 

 contraction of the diaphragm. The existence of this tonus may be easily 

 shown by dividing the phrenic nerves, when the lever at once sinks to 

 the base line of total relaxation. 



2. Effects of division of the vagi. Division of the vagi has been 

 performed by every physiologist who has investigated the innervation 

 of respiration, but with varying results. The typical effect, as described 

 by Eosenthal, I have already described. In many cases, however, the 

 issue may be quite different. The breathing may become irregular, and 

 accompanied with restlessness of the animal, while in other cases 

 division may be followed by long expiratory pauses. ISTow, in all 

 instances where the nerves are divided by section or ligature and 



1 Journ, PhysioL, Cambridge and London, 1889, vol. x. pp. 1-70, and pp. 279-290. 



