276 RESPIRATION 



The Regulation of the Respiration through the Afferent Vagus 

 Fibres. When one vagus is divided, there is little or no change in 

 the respiratory movements. Half an inch of one vagus nerve has 

 been excised in removing a tumour, and the patient showed no 

 symptoms whatever. But section of both vagi in such animals as 

 the dog, cat and rabbit causes respiration to become much deeper 

 and slower, the one change for a time compensating the other, so 

 that the total amount of air taken in and given out, the amount of 

 carbon dioxide eliminated, and the partial pressure of that gas in 

 the pulmonary alveoli are not greatly altered. The relative dura- 

 tion of the two respiratory phases is completely changed, inspira- 

 tion being much more prolonged than expiration. It has been 

 shown that the effect is really due to the loss of impulses that nor- 

 mally ascend the vagi, not to any irritation of the cut ends. For a 

 nerve can be frozen without exciting it ; and when a portion of each 

 vagus is frozen, the respiration is affected in precisely the same 

 way as when the nerves are divided. 



After section of both vagi certain fibres coming from the brain 

 above the respiratory centre appear to take a share in the regulation 

 of the respiratory movements. The bloodvessels supplying these 

 fibres, or the centres from which they come, can be blocked by 

 injection of paraffin wax into the common or internal carotid, or 

 the bulb can be severed with the knife above the level of the re- 

 spiratory centre, without any effect being produced upon the breath- 

 ing, except that the rate is as a rule somewhat lessened. But 

 when both the vagi and these upper paths are cut the character of 

 the respiration is changed, exceedingly prolonged inspiratory 

 spasms alternating with long periods of complete relaxation of the 

 diaphragm till the animal dies. 



From these facts it appears that the periodic 



of thft respiratory centre are being continually controlled and modi- 

 fied by impulses passing up the vagus, and that in the absence of 

 "tnese impulses a certain degree of control is exercised by the higher 

 paths, which, however, do not appear to be normally in action, at 

 any rate to the full measure of their capacity. When the vagi are 

 severed, the control of the higher paths comes into play, and is 

 sufficient still to keep the breathing regular, although it is slowed. 

 When the higher paths are cut off, the vagus of itself is able to regu- 

 late the discharge. But when both are gone, the respiratory centre, 

 freed from nervous control, passes into a condition of alternate 

 spasm and exhaustion. Of the central connections of these upper 

 paths but little is surely known. The corpora quadrigemina, how- 

 ever, seem to contain centres which can affect _the respiration. 

 Certain areas on the cerebral cortex have also been described, the 

 excitation of which modifies the respiratory movements. There is 

 no question that the cortex is connected, and extensively connected, 



