CHAP, ii.j RESPIRATION. 361 



respiratory centre into such a condition that it takes a much 

 longer time for the succeeding respiratory impulses to become 

 irresistible. A similar but even more pronounced condition may 

 be brought about in an animal by making it inspire oxygen, 

 or breathe ordinary air more rapidly and more forcibly than the 

 needs of the economy require. If in a xabbit artificial respiration 

 is carried on very vigorously for a while, and then suddenly 

 stopped, the animal does not immediately begin to breathe. For 

 a variable period no respiration takes place at all, and when it does 

 begin occurs gently and normally, only passing into dyspnoea if the 

 animal is unable to breathe of itself; and even then the transition is 

 quite gradual. Evidently during this period the respiratory centre 

 is in a state of complete rest, no explosions are taking place, no 

 respiratory impulses are being generated, and the quiet transition 

 from this condition to that of normal respiration shews that the 

 subsequent generation of impulses is attended by no great dis- 

 turbance. The cause of this state of things, which is known as 

 that of apncea, is to be sought for in the condition of the blood. 

 By the increased vigour of the artificial respiratory movements the 

 hemoglobin of the arterial blood, which in normal breathing is not 

 quite saturated, becomes almost completely so, and the quantity of 

 oxygen simply dissolved is increased, its tension being largely 

 augmented. Respiration is arrested because the blood is more 

 highly arterialized than usual. Thus we have in apnoea the 

 converse to dyspnoea; and both states point to the same con- 

 clusion, that the activity of the respiratory centre is dependent on 

 the condition of the blood, being augmented when the blood is less 

 arterial and more venous, being depressed when it is more arterial 

 and less venous than usual. 



The question now arises, Does this condition of the blood affect 

 the respiratory centre directly, or does it produce its effect by 

 stimulating the peripheral ends of afferent nerves in various parts 

 of the body, and, by the creation there of afferent impulses, 

 indirectly modify the action of the centre ? Without denying the 

 possibility that the latter mode of action may help in the matter, 

 as regards not only the vagi, but all afferent nerves, it is clear from 

 the following reasons that the main effect is produced by the 

 direct action of the blood on the respiratory centre itself. If the 

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

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

 increased activity of the Tespiratory 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 

 blood-vessels of the neck, dyspnoea is produced, though the opera- 

 tion produces 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 proto- 



