THE PNEUMOGASTEIC NERVE. 561 



and muscular movements in the larynx, that the pneumo- 

 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. 516); 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 

 recurrent nerve is not generally fatal, and that of both the 

 pneumogastric trunks is not always fatal (J. Reid), and, 

 when it is so, the death ensues slowly. This difference is, 

 probably, because the yielding of the cartilages of the 

 larynx in young animals permits the glottis to be closed 

 by the atmospheric pressure in inspiration, and they are 

 thus quickly suffocated unless tracheotomy be performed 

 (Legallois). In old animals, the rigidity and prominence 

 of the arytenoid cartilages prevent the glottis from being 

 completely closed by the atmospheric pressure ; even when 

 all the muscles are paralyzed, a portion at its posterior 

 part remains open, and through this the animal continues 

 to breathe. Yet the diminution of the orifice for respira- 

 tion may add to the difficulty of maintaining life. 



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