RESPIRATORY REFLEXES 327 



by a good singer. Respiratory movements can be accelerated or retarded by 

 electrical stimulation of the motor cortex of the dog and cat. The result, 

 according to F. Franck, does not depend upon the place of stimulus, but upon 

 its strength: strong stimulus giving a retardation, weak stimulus an acceler- 

 ation. The depth of respiration also is changed in one direction or the other. 

 These parts of the brain act only through the mediation of the respiratory 

 center in the medulla ; the fibers running from them to the center are there- 

 fore to be regarded as afferent pathways. The warrant for this view lies in 

 the fact already mentioned, that the respiratory movements continue after 

 section above the medulla. Moreover, it is not to be denied that some of the 

 results just discussed can be obtained by stimulation of the conducting path- 

 ways. The so-called brain centers for respiration seem therefore to represent 

 only pathways to the center in the medulla. We shall see immediately that 

 these paths and certain parts of the brain are, under certain circumstances, 

 of great service. 



3. RESPIRATORY REFLEXES 



Like all the other more complicated processes of the body the respiratory 

 movements are influenced by all possible kinds of afferent nerves. But there 

 are two of these paths more important than the rest, namely (1) the vagus, 

 and (2) the fibers which connect the higher parts of the brain with the 

 respiratory center. These accordingly we must consider first. 



A. REFLEXES THROUGH THE VAGI 



Notwithstanding the voluminous literature that has accumulated on the 

 influence of the afferent vagus fibers, our knowledge of their action on respira- 

 tion is still very meager. The statements of authors as to the facts bearing 

 on even the most important points differ considerably, and we can therefore 

 present the action of the vagus on respiration only in the broadest outline. 



Generally speaking, in the investigation of the influence of any nerve on 

 the processes of the body one obtains the best results by direct stimulation 

 of the nerve. Unfortunately this is not the case with the pulmonary vagus, 

 for section of the nerve is followed by much more profound effects than its 

 stimulation. 



A nerve cannot be cut with a pair of scissors without at the same time 

 stimulating it. Besides, an electric current (demarcation current, see page 48) 

 is set up in a cut nerve, and this may possibly exercise a stimulating influence. 

 Gad has shown, however, that the pulmonary vagi can be thrown out of action 

 without stimulation by cooling them sufficiently. For this purpose the vagi are 

 laid upon silver tubes which are filled with a cold mixture (e. g., a solution of 

 ammonium nitrate in water). 



Even under such circumstances different authors have not obtained per- 

 fectly harmonious results, although all are agreed that after bilateral blocking 

 of the vagus (1) the respiratory frequency falls, (2) that the inspirations 

 become deeper and (3) that the summit of inspiration shows a pause of 

 greater or less length (Fig. 129). But with respect to expiration after double 



