THE PNEUMOGASTRIC NERVE. 441 



3. It is partly through their influence on the sensibility and 

 muscular movements in the larynx, that the pneumogastric 

 nerves exercise so great an influence on the respiratory pro- 

 cess, 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, ex- 

 periments. 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 

 pneumogastric nerves are the principal conductors of the im- 

 pression of the necessity of breathing to the medulla oblon- 

 gata. 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 imperfectly 

 aerated blood flows (see p. 407): yet the respiration being re- 

 tarded, 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 per- 

 mits the glottis to be closed by the atmospheric pressure in in- 

 spiration, and they are thus quickly suffocated unless trache- 

 otomy 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 respiration may 

 add to the difficulty of maintaining life. 



In the case of slower death, after division of both the pneu- 

 mogastric nerves, the lungs are commonly found gorged with 

 blood, oadematous, or nearly solid, or with a kind of low pneu^ 

 monia, and with their bronchial tubes full of frothy bloody 

 fluid and mucus, changes to which, in general, the death may be 

 proximately ascribed. These changes are due, perhaps in part, 

 to the influence which the pneumogastric nerves exercise on the 

 movements of the air-cells and bronchi ; yet, since they are not 

 always produced in one lung when its pneumogastric nerve is 

 divided, they cannot be ascribed wholly to the suspension of 

 organic nervous influence (J. Reid). Rather, they may be 



