CHAP, ii.] KESPIKATIOK 485 



rial) the blood, the greater is the activity of the respiratory 

 centre. When by reason either of any hindrance to the entrance 

 of air into the chest, or other interference with the due inter- 

 change between the blood and the pulmonary air or of a greater 

 respiratory activity of the tissues, as during muscular exertion, 

 the blood becomes less arterial, more venous, i.e. with a smaller 

 charge of oxygen and more heavily laden with carbonic acid, 

 the respiration from being normal becomes laboured. We may 

 speak of normal breathing as eupnoea, and say that this, when 

 the blood is insufficiently arterialized, passes into dyspnoea, an 

 intermediate stage in which the respiratory movements are 

 simply exaggerated being known as hyperpnoea. The modifi- 

 cations of breathing thus caused by deficient arterialization of 

 blood are especially characterized by an increase in the total 

 energy of the respiratory impulses generated, and in this 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 dysp- 

 noea 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, over- 

 flows 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 res- 

 piratory 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 has- 

 tened as to catch each other up, and so to produce an inspirator}^ 

 tetanus like that resulting from stimulation of the vagus. On 



