330 RESPIRATION 



of both vagi, although it is very often unable to do so. At any rate, it appears 

 from these facts that the paths coming from the brain are, under certain cir- 

 cumstances, of very great importance for respiration, and, what is more, that 

 they act in the same way as the vagi nerves i. e., to inhibit inspiration. Expla- 

 nation of these phenomena is rendered more difficult because we cannot tell yet 

 to what extent they depend upon the stimulus given at the time the section is 

 made, and to what extent upon the mere disconnection of the pathways. 



In view of the profound influence of these brain pathways over the move- 

 ments of respiration, it is not difficult to understand why the severance of the 

 vagi is not always accompanied by the same results; for the effect must depend 

 largely upon the state of excitability of these brain paths at the time. For 

 example, it is possible that the shortening of expiration and the tonic contrac- 

 tion of the inspiratory muscles mentioned by Gad (but not observed by others), 

 was due to a stronger narcosis, on account of which the influence of the pathways 

 in the brain was somewhat weaker than in the investigations of the other authors. 



C. OTHER RESPIRATORY REFLEXES 



The respiratory movements are influenced in one way or another by other 

 afferent nerves. Among these the nerves of the respiratory passages are of 

 the greatest interest, because through them certain reflexes are discharged 

 which are of great service as a means of protection to the lungs. 



The nasal openings and the mucous membrane of the nasal passages re- 

 ceive their sensory fibers from the trigeminal nerve. Stimulation of this 

 nerve retards respiration. When the mucous membrane of the nose is stimu- 

 lated at the external openings or on the anterior or posterior end of the middle 

 and lower turbinated bones., or on the corresponding parts of the septum, 

 retardation or expiratory standstill, or even sneezing, results, according to the 

 strength and the kind of stimulus employed. Sneezing may also be abortive 

 i. e., only the first phase of it, the deep inspiration, may occur. Expiratory 

 standstill may also be induced by stimulation of the skin of the face under 

 certain circumstances, as when an animaPs head is submerged in water 

 (Fratschmer). 



Inasmuch as these reflexes prevent the entrance of foreign bodies or of 

 noxious vapors into the wider respiratory passages, the afferent nerves of the 

 larynx, particularly the superior laryngeal, serve to protect the deeper respir- 

 atory passages from foreign bodies. With feeble stimulation of the superior 

 laryngeal slowing of respiration and prolongation of the expiratory pause are 

 obtained ; on this account the individual respirations become deeper and longer. 

 With stronger stimulation one may obtain expiratory standstill or active ex- 

 piratory movements. Inspiratory spasms can be stopped by stimulation of the 

 superior laryngeal. 



The coughing reflex is discharged principally by the same nerve. The 

 statements of authors do not agree entirely as to the places in the larynx 

 and trachea from which the reflex is produced. 



We have the following statements concerning the effect of other afferent 

 nerves. Stimulation of the olfactory by an actual odor either slows or quickens 

 respiration or gives an expiratory pause. Stimulation of the optic by electricity 

 or by light has an accelerating effect on inspiration. The auditory nerve has 



