ACTION OF AFFERENT STIMULI ON THE RESPIRATORY RHYTHM 311 



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 to certain other muscles during expiration. 



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 ex- 

 piration 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 ver- 

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

 while on the opposide 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 as to the exact working 

 of the mechanism in its reactions. It is thought that the center may act 

 automatically, but normally its automatic discharges of nerve impulses are 



p 



If? R. 96 per 



BOTH VAGI CUT 





JYW 



'^ 



R. 120 



FIG. 240. The Effect on the Respiratory Rate of Cutting Both Vagi in the Dog. 

 The rate of 60 respirations per minute before the section of the nerves drops to 8 per 

 minute afterward. The arterial blood-pressure is also shown, the pressure in mm. mer- 

 cury is shown in the scale to the left. Compare with figure 182. 



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, figure 240. This may be the case, but to a less 

 marked degree, if one of the nerves is divided instead of both. If the cen- 

 tral, end of the divided nerve be stimulated with a weak but properly adjusted 



