562 THE CRANIAL NERVES. 



On the other hand, if the pneumogastric nerves be cut off, the customary 

 impression is no longer conveyed from the lungs to the medulla, and 

 the movements of respiration are consequently retarded. The imperfect 

 aeration of the blood thus induced reacts in turn upon the medulla, as 

 well as upon the other nervous centres, diminishing its sensibility, and 

 rendering it less able to respond to impressions of any kind. Thus the 

 difficulty, which consists in a want of the nervous reaction necessary 

 for respiration, increases from hour to hour, the breathing becomes con- 

 stantly more imperfect and sluggish, and at last ceases altogether. The 

 alteration in the tissue of the lungs, their engorgement and solidifica- 

 tion, add to the difficulty in aeration of the blood, and probably have, 

 at last, a considerable share in producing the fatal result. 



It is evident, however, that the pneumogastric nerves, although the 

 principal means for conveying to the medulla the stimulus for respira- 

 tion, are not the only ones. If they were so, respiration would stop 

 instantly after section of these nerves, as it does after destruction of the 

 medulla itself. The lungs are, no doubt, especially sensitive to the want 

 of oxygen and accumulation of carbonic acid in the blood ; and the 

 nervous impression thus produced is accordingly first felt in them. 

 There is reason to believe that all the vascular organs are more or less 

 capable of originating this impression, and that all the sensitive nerves 

 are capable, to some extent, of transmitting it. Although the first dis- 

 agreeable sensation, on holding the breath, makes itself felt in the 

 lungs, yet if we persist in suspending respiration, the feeling of dis- 

 comfort soon spreads to other parts ; and at last, when the accumula- 

 tion of carbonic acid has become excessive, all parts of the body are 

 pervaded by a general feeling of distress. It is easy, therefore, to under- 

 stand why respiration should be retarded, after section of the pneumo- 

 gastrics, since the chief source of the stimulus to respiration is cut off; 

 but the movements still go on, though more slowty than before, because 

 the other sensitive nerves, which continue to act, are in some measure 

 capable of conveying a similar impression. 



In order that the movements of respiration should go on with the 

 requisite frequency to maintain the aeration of the blood, it is necessary 

 that the pnenmogastric nerves, which are especially endowed with this 

 kind of sensibility, retain their integrity as nervous conductors between 

 the lungs and the medulla oblongata. In this function, they act alto- 

 gether as sensitive nerves; while the muscles of respiration receive 

 their reflex motor stimulus by way of the spinal nerves. 



Connection with the Respiratory Movements of the Glottis. The 

 respiratory movements of the glottis, already described in a former 

 chapter (p. 277) are essential parts of the mechanism of respiration. 

 They consist in the active opening of the glottis in inspiration, followed 

 by its partial collapse at the time of expiration. The opening of the 

 glottis, which is requisite for the free admission of air into the trachea, 

 is effected by the action of the posterior crico-arytenoid muscles. These 

 muscles, in contracting, rotate the arytenoid cartilages outward, and 



