THE CRANIAL NERVES. 485 



no effusion into the pleural cavity. The lungs are simply engorged 

 with blood, and, to a considerable extent, empty of air. 



The inference from these phenomena is that the pneumogastric nerves 

 are the channels for a sensitive impression from the lungs to the me- 

 dulla oblongata which excites, by reflex action, the movement of respira- 

 tion. Consequently when they are divided, their impression being no 

 longer conveyed to the nervous centres, the reflex act in the medulla 

 lacks its usual stimulus, and the movements of respiration diminish in 

 frequency. They do not cease altogether, because a similar impression 

 comes from other parts of the circulatory system. But the lungs are 

 the organs most directly concerned in respiration, and the most sensi- 

 tive to its deficiency ; and when their influence is cut off, the greater 

 part of the normal respiratory stimulus is wanting. The medulla, 

 accordingly, reacts less frequently, and the movements of respiration 

 are performed at longer intervals. 



This appears to be the only explanation which will account for the 

 immediate effects of dividing the pneumogastric nerves. The infre- 

 quency of respiration which follows directly upon this operation is not 

 due to paralysis of the respiratory muscles. It is not accompanied by 

 dyspnoea, nor by any sign of distress from defective respiration. It is 

 evident that the animal does not feel the need of breathing as under 

 ordinary conditions, and consequently makes no effort to compensate 

 for the loss. If respiration were reduced in frequency, the pneumogas- 

 tric nerves remaining entire, a sense of suffocation would soon be mani- 

 fest. This happens when the breath is voluntarily suspended ; the 

 sensation of discomfort being first perceptible in the lungs, but after- 

 ward extending over the whole system, and assuming the character 

 of an intolerable distress. When breathing is renewed, the unpleasant 

 sensation disappears, as inspiration renovates the air in the pulmonary 

 cavities. The impression transmitted by the pneumogastric nerves to 

 the medulla is sufficient to maintain respiration at its normal frequency. 

 When this impression is cut off, the rate of respiration is lowered nearly 

 one-half. 



But the subsequent changes after this operation are due to other 

 causes. When the pneumogastric nerves are divided in the middle of 

 the neck, the fibres of the inferior laryngeal nerve are involved in the 

 section. This paralyzes the laryngeal muscles, including those which 

 separate the vocal chords and open the glottis at the moment of inspi- 

 ration (page 238). The glottis is then left in a condition of flaccidity, 

 and instead of opening in inspiration for the admission of air, it col- 

 lapses and obstructs the passage. The quantity of air entering the 

 lungs is thus diminished, and the aeration of the blood still further 

 impaired. This no doubt causes the general sluggish condition of the 

 nervous system after section of the pneumogastrics. The medulla par- 

 ticipates in this derangement. It becomes less sensitive to the respira- 

 tory stimulus ; and as the stimulus itself is diminished, these conditions 

 react upon each other, and increase the difficulty of respiration. Thus 



