THE CONTROL OF THE RESPIRATION 351 



slowing of the breathing. Other experimental evidence of the existence 

 of cerebral respiratory fibers is furnished by cerebral localization experi- 

 ments. During stimulation of the cerebral cortex, for example, a marked 

 effect on the respiratory movements is often noted. 



Respiratory rhythm, unlike that of the heart, has often to be modified 

 in order that the respiratory mechanism may be used for other purposes 

 than the ventilation of the lungs. This alteration in rhythm may take 

 the form of a mere inhibition, such as the act of swallowing; or the 

 respiration may be altered, as in phonation and singing. More consid- 

 erable alteration in the expiratory discharge occurs in coughing and 

 sneezing, and still more in the acts of micturition, defecation, and parturi- 

 tion. We must conclude therefore that the rhythmic stimuli sent out 

 from the respiratory center are so weak that stimuli from other sources 

 may instantly inhibit or change their form at any stage of the cycle. 



Stimulation of the endings of the glossopharyngeal nerve inhibits res- 

 piration, which explains the holding of the breath that occurs in swal- 

 lowing. 



The superior laryngeal branch of the vagus has an occasional influence 

 on the respiratory center, its particular function being in connection with 

 the act of coughing. When a foreign body irritates the mucous membrane of 

 the larynx, the nerve fibers transmit impulses to the respiratory center 

 which excite a violent expiration and at the same time cause the glottis to 

 close. The closure of the glottis lasts, however, only during the first part 

 of the expiration; it then opens, with the result that the sudden release 

 of intrapulmonic pressure causes the expulsion of the foreign substance 

 from the air passages. Sneezing is a similar mechanism, being due to 

 stimulation of the endings of the fifth nerve in the nasal mucosa, the 

 only difference being that the initial obstruction to the forced expira- 

 tion is higher up. 



