THE NERVOUS MECHANISM OF RESPIRATION. 381 



resulting from interference with the nervous arrangements such as those fol- 

 lowing upon section of the vagus nerves, in which case, as we have seen, the 

 rhythm is much more profoundly affected than the amount. In dyspnoea 

 the breathing is frequently quicker as \vell as deeper, there is an increase in 

 the sum of efferent respiratory impulses, and the expiratory impulses, which 

 in normal respiration are very slight, acquire a pronounced importance. As 

 the blood becomes in cases of obstruction less and less arterial, more and 

 more venous, the discharge from the respiratory centre becomes more and 

 more vehement, and instead of confining itself to the usual tracts and pass- 

 ing down to the ordinary respiratory muscles, overflows into other tracts and 

 puts into action other muscles, until there is, perhaps, hardly a muscle in the 

 body which is not made to feel its effects. The muscles which are thus more 

 and more thrown into action are especially those tending to carry out or to 

 assist expiration ; and at last, if no relief is afforded, the violent but still 

 definite respiratory movements give way to general convulsions of the whole 

 body, which, however, have to a certain extent an expiratory character. 

 With the onset of these convulsions dyspnoea is said to have passed into 

 asphyxia. By the violence of these convulsions the whole nervous system 

 becomes exhausted, the convulsions cease, and death is ushered in through a 

 few infrequent and long-drawn breaths; but to this matter we shall return. 

 The effect of venous blood, then, is to augment all those natural explosive 

 decompositions of the substance of the central nervous system which give 

 rise to respiratory impulses ; it increases their amount and also quickens their 

 rhythm. The latter change, however, is much less marked than the former, 

 the respiration being much more deepened than hurried, and the several 

 respiratory acts are never so much hastened as to catch each other up, and 

 so to produce an inspiratory tetanus like that resulting from stimulation of 

 the vagus. On the contrary, especially as exhaustion begins to set in, the 

 rhythm becomes slower than proportionate to the weakening of the individual 

 movements. 



314. The question naturally arises, Does this condition of the blood 

 affect the substance of the central nervous system, that is to say, the respira- 

 tory centre in the medulla (and the subsidiary spinal nervous mechanisms) 

 directly, or does it produce its effect by stimulating the peripheral ends of 

 afferent nerves in various parts of the body, and, by the generation there of 

 afferent impulses, indirectly modify the action of the central nervous system? 

 Without denying the possibility that the latter mode of action may help in 

 the matter, as regards not only the vagus, but all afferent nerves, the follow- 

 ing facts seem to show that the main effect is produced by the direct action 

 of the blood on the central nervous system, and, indeed, on the medullary 

 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 respiratory centre, as shown by the 

 increased vigor of the facial respiratory movements. If the supply of blood 

 be cut off from the medulla by ligature of the carotid and intervertebral 

 arteries dyspnoea is produced, though the operation produces 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, shows 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 



