r 



538 KESPIRATION 



begin with, however, it may be well to state that currents of moderate 

 strength invariably evoke a respiratory standstill in the inspiratory 

 position. This phenomenon is practically identical with that observed 

 upon stimulation of the intact superior laryngeal nerve, or of its 

 central end. Very weak stimuli are prone to develop expiratory 

 tendencies which are usually accompanied by an inhibition of the 

 inspiratory movements. With strong currents the results are per- 

 plexing, although it is quite evident that they consist essentially 

 in a respiratory cessation with the chest either in the inspiratory 

 or expiratory position. It need scarcely be mentioned that these 

 effects may also be evoked by the stimulation of the intact vagus. 



The Self -regulation of Respiration. — The foregoing experimental 

 data show very clearly that the division of the vagi nerves prevents 

 certain stimuli from reaching the respiratory center which originate 

 along the pulmonary passage and ordinarily tend to increase the activity 

 of these ganglion cells. When no longer under the influence of these 

 afferent impulses, the center falls back upon its inherent automaticity, 

 which gives rise to regular but relatively infrequent impulses. In 

 the second place, it must be concluded that the vagus nerve em- 

 braces two kinds of afferent fibers, or rather, afferent fibers which are 

 capable of conducting two types of impulses. One of these inhibits 

 inspiration and the other expiration. Accordingly, it may be con- 

 jectured that the inhibition of the inspiratory muscles allows the 

 development of the expiratory process, while the inhibition of the 

 expiratory muscles favors the occurrence of inspiration. 



In accordance with this exposition Hering and Brener^ have 

 formulated the hypothesis that the respiratory movements regulate 

 themselves; i.e., every expiration incites an inspiration and every 

 inspiration an expiration. The vagi, therefore, are regarded as form- 

 ing the most important link in a check-system which insures a proper 

 sequence and depth of the successive respiratory movements. This 

 leads to a much greater frequency of the respiratory movements than 

 could be obtained if the center alone were the controlling agent. The 

 latter, as has been shown above, possesses a slow rate of discharge. 

 When the lungs are expanded, a stimulus is set up in these organs 

 which travels over the inspiratory fibers of the vagus and eventually 

 stops this movement, permitting expiration to set in. Quite similarly, 

 the deflation of the lungs reflexly incites the subsequent inspiration. 

 Whether these intrapulmonic stimuli are chemical or mechanical 

 in their nature is a much debated question. It seems, however, that 

 the mechanical ones are the most important. They find their origin 

 in the alternate stretching of the vagal terminals which may be 

 imagined to invest the bronchial tubes in the manner of calipers. 

 It should be remembered, however, that the expiratory process is a 

 passive phenomenon and is not associated under ordinary conditions 

 with a contraction of the respiratory muscles, and hence, the inhibiting 

 1 Sitzungsb. der Wiener Akad. der Wissensch., cviii, 1868, 909. 



