THE NERVOUS REGULATION OF RESPIRATION 



535 



.^^B^ 



matter of common experience that the entrance of a foreign body into 

 the larynx causes an immediate inhibition of inspiration and a forced 

 expiration, the air being ejected in this case through the mouth. It 

 need scarcely be repeated that the impulses generated in this region 

 of the respiratory passage, are first conducted through the nuclei of 

 the vagi nerves to the respiratory center, 

 whence the efferent discharges are di- 

 verted to the different muscles of respira- 

 tion. Obviously, the division of either 

 the right or left superior laryngeal nerve 

 must render the corresponding side 

 of the larynx insensitive to stimulation. 

 Furthermore, inasmuch as this nerve is 

 the only sensory nerve of this organ, the 

 division of both nerves must lead to a 

 complete paralysis of sensation. An 

 animal cannot long survive this proced- 

 ure, because the gradual accumulation 

 of foreign substances in the upper res- 

 piratory passage finally involves the lung 

 tissue and gives rise to an inflammatory 

 reaction which bears the essential char- 

 acteristics of pneumonia. 



While discussing this subject, it might 

 be well to mention that the superior 

 laryngeal nerves are not entirely sensory 

 in their function, but also embrace a 

 number of efferent fibers which innervate 

 the cricothyroid muscles (Fig. 263). 

 Keeping these facts clearly in mind, it 

 will, therefore, be seen that the stimula- 

 tion of the intact superior laryngeal nerve 

 must produce impulses which (a) pursue 

 an afferent course and give rise to an in- 

 hibition of inspiration and a forced ex- 

 piration, and (6) pass in an efferent direc- 

 tion to cause a contraction of the corre- 

 sponding cricothyroid muscle. Accord- 

 ingly, the division of this nerve must in- 

 duce a loss of sensation on the side of 



the injury, as well as a paralysis of the corresponding cricothyroid 

 muscle. The stimulation of the distal end of the divided nerve then 

 gives a contraction of the cricothyroid muscle, while the excitation 

 of its central end elicits those sensations which ordinarily produce an 

 inspiratory standstill and forced expiratory blasts of air. 



The larynx also receives a second nerve supply by way of the 

 inferior laryngeal branches of the vagus (Fig. 263). Since these nerves 



Fig. 263. — The Innekvation 

 or THE Labynx (Posterior View; 

 One Side). 



B, base of tongue; E, epi- 

 glottis; A, arytenoid muscles; 

 CA, crico-arytenoid muscle; T, 

 trachea; V, vagus nerve; SL, 

 superior laryngeal nerve; J and 

 O, its inner and outer branches; 

 JL, inferior laryngeal nerve; Br, 

 vagal fibers innervating bron- 

 chial musculature. 



