THE PNEUMOGASTBIC NERVE. 561 



laryngeal branch, and excites contraction of the muscles 

 that close the glottis. Both these branches of the pneumo - 

 gastric co-operate also in the production and regulation 

 of the voice ; the inferior laryngeal determining the con- 

 traction of the muscles that vary the tension of the vocal 

 cords, and the superior laryngeal conveying to the mind 

 the sensations of the state of these muscles necessary 

 for their continuous guidance. And both the branches 

 co-operate in the actions of the larynx in the ordinary 

 slight dilatation and contraction of the glottis in the acts 

 of expiration and inspiration, and more evidently in those 

 of coughing and other forcible respiratory movements 



(P- 233). 



3. It is partly through their influence on the sensibility 

 and muscular movements in the larynx, that the pneutno- 

 gastric nerves exercise so great an influence on the respira- 

 tory process, and that the division of both the nerves is 

 commonly fatal. To determine how death is in these cases 

 produced, has been the object of innumerable, and often 

 contradictory, experiments. It is probably produced 

 differently in different cases, and in many is the result of 

 several co-operating causes. Thus, after division of both 

 the nerves, the respiration at once becomes slower, the 

 number of respirations in a given time being commonly 

 diminished to one half, probably because the pneumo- 

 gastric nerves are the principal conductors of the impres- 

 sion of the necessity of breathing to the medulla oblongata. 

 Respiration does not cease ; for it is probable that the 

 impression may be conveyed to the medulla oblongata 

 through the sensitive nerves of all parts in which the im- 

 perfectly aerated blood flows (see p. 517) ; yet the respira- 

 tion being retarded, adds to the other injurious effects of 

 division of the nerves. 



Again, division of both pneumogastric trunks, or of 

 both their recurrent branches, is often very quickly fatal in 

 young animals ; but in old animals, the division of the 



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