298 



RESPIRATION 



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 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 



BOTH VAGICUT 



R. 120 



"rrrrrrrrrrrrrrrrrr 



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 179. 



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 

 strength of interrupted current, the effect is that the respirations are quick- 

 ened, and if the stimuli are properly regulated the normal rhythm of respi- 

 ration may be resumed. If the stimuli be repeated with stronger currents, 

 the breathing is brought to a standstill, sometimes at the height of inspira- 

 tion, by tetanus of the diaphragm. Usually, however, stimulation of the 

 central end of the divided vagus produces still greater slowing than that 

 which follows the division, so that the respirations cease with the diaphragm 

 in a condition of complete relaxation, figures 205 and 241. 



