RESPIRATION 9 



It was found by Legallois and subsequent investigators that the 

 nervous connections both above and below the respiratory center 

 can be successively severed without preventing the rhythmic dis- 

 charges of inspiratory and expiratory impulses except in so far as 

 efferent nerves connected with the center are cut off from it. Thus 

 the rhythmic discharges of the center are not dependent on afferent 

 nervous impulses and continue regularly so long as normal arterial 

 blood is supplied to it. In this sense the action of the center is 

 automatic. On the other hand the mode of action of the center is 

 much affected by nervous stimuli. 



In the first place its action is to a large extent under voluntary 

 control. Thus the breathing can easily be suspended for about a 

 minute, and in the actions of speaking, singing, etc., is greatly 

 interfered with. The rate and depth of breathing are also 

 under voluntary control, and may be much affected by emotion. 

 Consciously perceived stimuli of all kinds may also affect the 

 breathing particularly stimuli affecting the air passages. The 

 irregularity and variability of the breathing owing to all these 

 causes tended to direct the attention of physiologists away from 

 the central problem of how the breathing responds to fundamental 

 physiological requirements. 



It was soon discovered that apart from consciously felt stimuli 

 the breathing is specially affected by afferent stimuli conducted 

 by the vagus nerve. Early last century it was noticed that when the 

 vagus nerves are severed the breathing becomes less frequent and 

 deeper; and on stimulating the vagi various marked effects, de- 

 pending on the strength of stimulus, were found to be produced 

 on the breathing. 



In 1868 Hering and Breuer 14 made the striking discovery that 

 on mechanically interrupting, at the end of inspiration, the ex- 

 pulsion of air from the lungs the rhythm of respiratory effort is 

 interrupted for a time, until at last this interruption is broken by 

 an inspiratory effort, followed by alternating expiratory and in- 

 spiratory efforts showing that the center has renewed its rhythmic 

 activity. Similarly if at the end of expiration air is prevented from 

 entering the lungs there is an interruption before the center re- 

 turns to its normal rhythmic activity. These effects are completely 

 absent if the vagi have been divided. The slow rhythmic dis- 

 charges of the center go on quite independently of whether the 

 inflation or deflation of the lungs is prevented or not. 



"Hering and Breuer, Sitzber. d. Wiener Akad., Math-natural. Cl. (2), LVII, 

 p. 672 and LVIII, p. 909, 1868. 



