CHAP. IL] RESPIRATION. 631 



nerves, the following facts seem to shew that the main effect is 

 produced by the direct action of the blood on the central nervous 

 system and indeed on the bulbar respiratory centre itself. If the 

 spinal cord be divided below the spinal bulb, and both vagus nerves 

 be cut, want of proper aeration of the blood still produces an in- 

 creased activity of the respiratory centre, as shewn by the increased 

 vigour of the facial respiratory movements. If the supply of blood 

 be suddenly cut off from the bulb by ligature of the carotid and 

 vertebral arteries dyspnoea is produced, though the operation pro- 

 duces at first no change in the blood generally, but simply affects 

 the respiratory condition of the bulb itself by cutting off its 

 blood-supply, the immediate result of which is an accumulation 

 of carbonic acid and a paucity of available oxygen in the nervous 

 substance of that region. If the blood in the carotid artery in an 

 animal be warmed above the normal, a dyspnoea is produced which, 

 though apparently not quite identical with the dyspnoea caused 

 by imperfect arterialization of the blood, shews that the too high 

 temperature of the blood directly affects the activity of the 

 respiratory centre. But the most conclusive argument is furnished 

 by the following so-called " cross circulation " experiment. In two 

 animals (dogs) the central portions of both carotids of the one 

 animal are connected by means of tubes with the peripheral por- 

 tions of the carotids of the other animal, and the peripheral portion 

 of a jugular of the second animal is similarly connected with the 

 central portion of a jugular of the first animal. Both vertebral 

 arteries, the ascending cervical arteries and the other jugulars of 

 the second animal are then tied. Under these conditions the brain 

 of the second animal is supplied entirely by blood from the first 

 animal ; its own blood is completely shut off from its brain, and 

 the blood of the first animal, ' the feeder,' supplies the brain of 

 the second, the 'fed' animal, but no other part. Precautions 

 having been taken against the clotting of blood in the tubes 

 making the junctions, the two animals may be kept in this con- 

 dition for more than an hour. If now the respiratory interchange 

 of the 'feeder' be interfered with, the 'fed' animal makes dyspnoeic 

 movements, whereas interference with its own respiratory inter- 

 change does not produce this effect ; it is the venous blood of the 

 ' feeder ' brought to bear on the bulbar respiratory centre of the 

 ' fed ' which calls forth the exaggerated and modified respiratory 

 movements. We may conclude therefore that the condition of 

 the blood affects respiration by acting directly on the respiratory 

 centre. Moreover it is the medullary centre which, at all events 

 in adult animals, is affected by the too venous blood, since after 

 division of the , spinal cord below the bulb, dyspnoeic thoracic 

 respiratory movements and convulsions do not follow upon ex- 

 clusion of air. They are however stated to occur in new-born 

 animals, indicating that the subsidiary mechanisms in the upper 

 spinal cord of which we spoke in 363 may be also affected 



