224 A MANUAL OF PHYSIOLOGY 



Section of one phrenic is followed by paralysis of the corre- 

 sponding half of the diaphragm, section of both phrenics by 

 complete paralysis of that muscle, and although respiration still 

 goes on by means of the muscles which act upon the ribs, it is 

 usually inadequate to the prolonged maintenance of life. In 

 the horse, however, not only has survival been seen after this 

 operation, but the animal, after the first temporary increase 

 in the frequency of the breathing had disappeared, could be 

 driven in a light vehicle without any marked dyspnoea. The 

 phrenic nuclei in the two halves of the cord are connected 

 across the middle line. For when a semisection of the cord is 

 made between this level and the respiratory centre in the medulla, 

 respiratory impulses are still able to reach both phrenic nerves. 

 In some animals both halves of the diaphragm go on contracting. 

 But when, as usually happens, this is not the case, and the 

 diaphragm on the side of the semisection has ceased to act, it 

 at once begins to contract again when the opposite phrenic 

 nerve is cut, and the respiratory impulse, descending from the 

 bulb, is blocked out from the direct, and forced to follow the 

 crossed path. It has been shown that the crossing takes place 

 at the level of the phrenic nuclei, and*nowhere else (Porter). 



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 pres- 

 sure of that gas in the pulmonary alveoli are not greatly altered. 

 The relative duration of the two respiratory phases is completely 

 changed, inspiration being much more prolonged than expiration. 

 It has been shown that the effect is really due to the loss of im- 

 pulses that normally 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 sup- 

 plying 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 respiratory centre, without any'effect being produced upon 

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



