RESPIRATION 213 



the case, and the diaphragm on the side of the hemisection 

 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 (Billroth). But 

 section of both vagi generally (though not always) causes re- 

 spiration to become for a time much deeper and slower, the 

 one change just compensating the other, so that the total 

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

 carbon dioxide eliminated, are not altered. Gad has shown 

 that the effect is really due to the loss of impulses 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. 



A similar change follows the blocking of the paths connect- 

 ing the respiratory centre with the brain above, by injection 

 of paraffin wax into the common or internal carotid. The 

 bloodvessels supplying the nerve-fibres which connect the 

 respiratory centre with the brain may in this way be closed 

 by artificial emboli. The nerves lose their function, as if 

 they had been cut ; no impulses now reach the respiratory 

 centre from above ; and the respiration becomes markedly 

 slowed and deepened, just as happens when the vagi are 

 divided. Where only the vagus or these 'higher paths,' 

 but not both, are cut off, the respiration remains regular, 

 although deep, and perhaps in course of time tends to 

 resume its original type. But when both paths are cut, the 

 character of the respiration is entirely changed ; periods of 

 rapid and spasmodic breathing alternate with periods of 

 complete cessation, till the animal dies (Marckwald). 



From these facts it appears that the periodic automatic 



