THE RESPIRATORY NERVE CENTER 273 



remainder of the central nervous system be separated from it, respiration 

 continues. It may be considered almost certain that the medullary center 

 is the only true respiratory center. Langendorff states that in newly born 

 animals in which the medulla has been immediately cut across at a level a 

 few millimeters below the point of the calamus scriptorius, respiration con- 

 tinues for some time, but this is questionable. Normal respiration does 

 not occur after separation of the bulb from the cord, and the so-called 

 respiratory movements noticed by Langendorff are merely tetanic contrac- 

 tions of the respiratory muscles in which often enough other muscles take 

 part. 



The action of the medullary center is to send out impulses during in- 

 spiration, which cause contractions of the inspiratory muscles a, of the 

 nostrils and jaws, through the facial and inferior division of the fifth nerves; 



b, of the glottis, chiefly through the inferior laryngeal branches of the vagi; 



c, of the intercostal and other muscles which produce raising of the ribs, 

 chiefly through the intercostal nerves, and d, of the diaphragm, through the 

 phrenic nerves. If any one of these sets of nerves be divided, respiratory 

 movements of the corresponding muscles cease. Similarly it may be supposed 

 that the center sends out impulses during expiration to certain other muscles. 

 It has been suggested, however, that the center is double, that it is made up 

 of inspiratory cells which are constantly in action, and of an expiratory group 

 of cells which act less generally, inasmuch as ordinary tranquil expiration 

 is seldom more than an elastic recoil, and not a muscular act to any marked 

 degree. 



The respiratory center is also bilateral, as has been proven by the method 

 of antero-posterior section of the medulla. The tracts from each half of the 

 center are separate and distinct. If the cervical cord be split into a right 

 and left half, and one side sectioned at the level of the second cervical verte- 

 bra, then the respiratory movements of that side of the diaphragm cease 

 while on the opposite side they continue their rhythm. 



Assuming this view of the quadruple nature of the respiratory centers 

 to be correct, there is some difference of opinion of the exact mode of action; 

 it is thought that the center may act automatically, but normally is influenced 

 by afferent impulses from the periphery, as well as by impulses passing down 

 from the cerebrum. The center is, in other words, both automatic and 

 reflex. It will be simplest to discuss its reflex function first. 



Action of Afferent Stimuli on the Respiratory Rhythm. Action 

 of the vagi. If both vagi be divided in the neck, the respirations become 

 much slower and deeper. This may be the case, but to a less marked degree, 

 if one of the nerves is divided instead of both. If the central end of the 

 divided nerve be stimulated with a weak but properly adjusted strength of 

 interrupted current, the effect is that the respirations are quickened, and if 

 the stimuli are properly regulated the normal rhythm of respiration may 

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