CHAP, ii.] RESPIRATION. 599 



respect differ from the modifications resulting from interference 

 with the nervous arrangements such as those following 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 well 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 

 passing 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 sub- 

 stance 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 out of proportion to the weakening of the individual move- 

 ments. 



371. The question naturally arises, Does this condition of the 

 blood affect the substance of the central nervous system, that is 

 to say, the respiratory 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 following facts seem to shew that the main effect is pro- 

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

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



