600 EFFECTS OF DEFICIENCY OF OXYGEN. [BOOK n. 



spinal cord be divided below the medulla oblongata, and both vagi 

 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 cut off from the medulla by ligature of the carotid and inter- 

 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 medulla 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. 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 medulla, dyspnceic 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 

 by the too venous blood ; but the doubts which we previously 

 urged hold good in these cases also. 



While the respiratory centre is thus being affected by the too 

 venous blood, it is, until exhaustion begins to set in, more irritable, 

 more easily and largely affected by afferent impulses than in its 

 normal condition. During dyspnoea a stimulus which applied to 

 the vagus or to some other sensory nerve under normal conditions 

 would produce little or no effect, may start very powerful respira- 

 tory movements. 



372. Deficient aeration produces two effects in blood : it 

 diminishes the oxygen, and increases the carbonic acid. Do both 

 of these changes affect the respiratory centre, or only one, and if 

 so, which ? When an animal is made to breathe an atmosphere 

 containing nitrogen only, the exit of carbonic acid by diffusion is 

 not affected, and the blood, as is proved by actual analysis, 

 contains no excess of carbonic acid. Yet all the phenomena 

 of dyspnoea . are present, and if the experiment be continued, 

 convulsions ensue and the animal dies in asphyxia. In this case 

 the result can only be attributed to the deficiency of oxygen. On 

 the other hand, if an animal be made to breathe an atmosphere rich 

 in carbonic acid, but at the same time containing abundance of 

 oxygen, though the breathing becomes markedly deeper and also 

 somewhat more frequent, there is no culmination in a convulsive 

 asphyxia, even when the quantity of carbonic acid in the blood, as 



