360 NERVOUS MECHANISM. [BOOK n. 



the respiratory centre is made to respond delicately to the varying 

 needs of the economy. 



Besides these nervous influences, however, there is another 

 circumstance which perhaps above all others affects the respiratory 

 centre, and that is the condition of the blood in respect to its 

 respiratory changes ; the more venous (less arterial) 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 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 oxy-hoemoglobin and more heavily laden with 

 carbonic acid, the respiration from being normal becomes laboured. 

 We may speak of normal breathing as eupncea, and say that this, 

 when the blood is insufficiently arterialized, passes into dyspncea, 

 an intermediate stage in which the respiratory movements are 

 simply exaggerated being known as hyperpncea. This effect of 

 deficient arterialization of blood is very different from that of 

 section of the vagi: it is no mere change in the distribution of 

 impulses ; the breathing is quicker as well as deeper, there is an 

 increase in the sum of efferent impulses proceeding from the centre, 

 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, puts into action other muscles, until there is 

 perhaps hardly a muscle in the body which is not made to feel it& 

 effects. And this discharge may, as we shall see in speaking 

 of asphyxia, continue till the nervous energy of the respiratory 

 centre is completely exhausted. The effect of venous blood then is 

 to augment these natural explosive decompositions of the nerve- 

 cells of the respiratory centre 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 in- 

 spiratory 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 movements. 



On the other hand, the blood may be made not more but less 

 venous than usual. When we attempt to hold our breath, we find 

 that we can only do this for a limited time; sooner or later a 

 breath must come; but the time during which we can remain 

 without breathing may be much prolonged, if we first of all take a 

 series of deep breaths. .By this increased ventilation we bring our 



