468 THE CRANIAL NERVES. 



spinal accessory, or any other one of the above-mentioned nerves, 

 might be affected by local accident or disease, it would be very 

 improbable that any single injury should paralyze simultaneously 

 the spinal accessory, the facial, the hypoglossal, and the first and 

 second cervicals. The respiratory movements of the larynx are 

 consequently much more thoroughly protected than those which 

 are merely concerned in the formation of the voice. 



Lungs. The influence of the pneumogastric upon the function 

 of the lungs is exceedingly important. The nerve acts here, as in 

 most other organs to which it is distributed, in a double or mixed 

 capacity ; but it is principally as the sensitive nerve of the lungs 

 that it has thus far received attention. It is this nerve which 

 conveys from the lungs to the medulla oblongata that peculiar 

 impression, termed besoin de respirer, which excites by reflex action 

 the diaphragm and intercostal muscles, and keeps up the play of 

 the respiratory movements. As we have already shown, this action 

 is an involuntary one, and will even take place when consciousness 

 is entirely suspended. It may indeed be arrested for a time by an 

 effort of the will ; but the impression conveyed to the medulla soon 

 becomes so strong, and the stimulus to inspiration so urgent, that 

 they can no longer be resisted, and the muscles contract in spite of 

 our attempts to restrain them. 



A very remarkable effect is accordingly produced on respiration 

 by simultaneous division of both pneumogastric nerves. This 

 experiment is best performed on adult dogs, which may be ether- 

 ized, and the nerves exposed while the animal is in a condition of 

 insensibility, avoiding, in this way, the disturbance of respiration, 

 which would follow if the dissection were performed while the ani- 

 mal was conscious and sensible to pain. After the effects of the 

 etherization have entirely passed off, and respiration and circulation 

 have both returned to a quiescent condition, the two nerves, which 

 have been previously exposed and secured by a loose ligature, may 

 be instantaneously divided, and the effects of the operation readily 

 appreciated. 



Immediately after the division of the nerves, when performed in 

 the above manner, the respiration is hurried and difficult, owing to 

 the sudden paralysis of the larynx and partial closure of the glottis 

 by the vocal chords, as already described. This condition, how- 

 ever, is of short continuance. In a few moments, the difficulty of 

 breathing and the general agitation subside, the animal becomes 

 perfectly quiet, and the only remaining visible effect of the opera- 



