312 RESPIRATION 



strength of interrupted current, the effect is to quicken the respirations. 

 And if the stimuli are properly regulated the normal rhythm of respira- 

 tion may be approached. If the stimuli be repeated with stronger currents, 

 the breathing is brought to a standstill, sometimes at the height of inspira- 

 tion, 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, figures 205 and 241. 



The sensory action of the vagus may therefore be to call forth either 

 inspiration or expiration the impulses passing up the vagi being factors 

 for the production and regulation of the normal variations in respiratory 

 rhythm. The fibers of the vagus are stimulated under the following cir- 

 cumstances: one set of fibers, those which tend to inhibit expiration and 

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

 lung tissue is under least tension, i.e., in a condition of expiration. The 

 fibers which tend to inhibit inspiration and to promote expiration are stim- 

 ulated when the lung is fully expanded. The afferent impulses by this view 

 are the results of mechanical stimulation, and do not depend upon the 

 chemical nature of the gases within the pulmonary alveoli. 



FIG. 241. The Effect of Stimulating the Vagus on Respiratory Rate. The stimulus 

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

 the stimulus is removed. Time in seconds. The intra-tracheal pressure is recorded 



The Respiratory 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 reaction is very 

 constant, respirations are slowed, and there is a distinct tendency toward 

 expiration, as shown by the contractions of the abdominal muscles. Thus 

 the superior laryngeal fibers inhibit inspiration and stimulate expiration, 

 while the deep branch of the vagus contains fibers which stimulate inspiration 

 and inhibit 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-pharyngeal 

 nerves produces no effect upon respiration, stimulation causes inhibition of 

 inspiration for a short period. This action accounts for the very necessary 



