ACTION OF OTHER SENSORY NERVES 299 



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 stimulated 

 under the following circumstances: those fibers which tend to inhibit expira- 

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

 when the lung is empty and in a condition of expiration. 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. 



on. of/ 



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 intratracheal 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 effect is very con- 

 stant 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 inspiration 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-pharyngeal 

 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 

 stimulation is only temporary, and is followed by normal breathing 

 movements. 



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

 fluenced strongly by afferent nerve impulses having their origin in general 

 sensory nerves, particularly the nerves of the skin. Cold water suddenly 

 dashed on the skin is followed by a deep inspiration. Stimulation of the 

 splanchnics or of the abdominal branches of the vagi produces expiration. 

 Stimulation of the isolated sciatic nerve of the dog or of the rabbit causes a 



