274 RESPIRATION 



be resumed. If the stimuli be repeated with stronger currents, the breathing 

 is brought to a standstill, sometimes at the height of inspiration, by tetanus 

 of the diaphragm. Usually, however, stimulation of the central end of the 

 divided vagus produces still greater slowing than that which follows the division 

 so that the respirations cease with the diaphragm in a condition of complete 

 relaxation. 



The action of the vagus may be to call forth either inspiration or expira- 

 tion the impulses passing up the vagi being necessary to the production 

 of the normal respiratory rhythm. The fibers of the vagus are used under 

 the following circumstances: those fibers which tend to inhibit expiration 

 and to stimulate inspiration are stimulated at their origin in the lung when the 



on- off 



FIG. 239. The Effect of Stimulating the Vagus Nerve on Respiratory Rate. The stimulus 

 was applied between the points " on " and " off." The inhibition lasts some seconds after the stim- 

 ulus is removed. Time in seconds. The intratracheal pressure is recorded. 



lung is empty and in a condition of expiration, and the fibers which tend 

 to inhibit inspiration and to promote expiration are stimulated when the 

 lung is fully expanded. The afferent impulses by this view are the results 

 of mechanical stimulation, and do not depend altogether upon the chemical 

 nature of the gases within the pulmonary alveoli. 



Action of the Superior Laryngeal Nerves. If the superior laryngeal 

 branch of the vagus be divided, which usually produces no apparent effect, 

 and the central end be stimulated, the effect is very constant, respirations 

 are slowed, but there is a tendency toward expiration, as is shown by the 

 contraction of the abdominal muscles. Thus, the vagus contains fibers 

 which stimulate inspiration and inhibit expiration, as well as other fibers 

 which have the reverse effect ; while the superior laryngeal fibers inhibit in- 

 spiration and stimulate expiration. 



The superior laryngeal nerves are true expiratory nerves, and are nor- 

 mally set in action when the mucous membrane of the larynx is irritated. 

 They are not in constant action like the vagi. 



Action of the Glosso-pharyngeal Nerves. It has been ascertained, 

 by the researches of Marckwald, that while division of the glosso-^)haryngeal 

 nerves produces no effect upon respiration, stimulation of them causes in- 

 hibition of inspiration for a short period. This action accounts for the very 

 necessary cessation of breathing during swallowing. The effect of the stimu- 

 lation is only temporary, and is followed by normal breathing movements. 



Action of Other Sensory Nerves. The respiratory center is in- 



