306 MANUAL OF PHYSIOLOGY. 



during expiration the left ventricle receives from the lungs and 

 ejects to the systemic arteries an amount of blood slightly in 

 excess of that which it receives and ejects during inspiration. 

 This may have a direct effect on the pressure in the great arte- 

 rial trunks. But it is more than probable that excess of blood 

 in the heart cavities acts as a nervous stimulus, and excites the 

 inhibitory centre of the heart and the depressor centres which 

 control the arterioles. 



The adoption of this view appears necessary from the follow- 

 ing facts : 



(1.) The rise in pressure is not exactly synchronous with 



expiration or inspiration ; 

 (2.) The heart beats more slowly during expiration than 



inspiration ; 



(3.) This difference at once disappears if the vagi be cut, 

 and the respiratory wave becomes greatly modified ; 

 (4.) The respiratory wave is observed when artificial respi- 

 ration is employed, in which the forcing of air into 

 the lungs is the cause, and not the result, of the 

 thoracic movements, so that the pressure effects are 

 reversed. 



We may conclude, then, that a sympathy in action can be dis- 

 tinctly recognized in the working of the respiratory and cardiac 

 nerve centres. 



Since the undulations known as Traube's curves occur in cura- 

 rized animals when no respiratory movements are performed, it 

 has been proposed to explain the respiratory undulations in the 

 same way, namely, by referring them to a stimulation of the 

 vasoinotor centre by impure blood, which by rhythmical impulses 

 increasing the contraction of the arterioles causes a rhythmical 

 variation in the blood pressure. This explanation seems to be 

 rendered unsatisfactory by the fact that the respiratory undula- 

 tions go on even when the arterioles are cut off from their chief 

 nerve centres by sections of the spinal cord. So that if these 

 undulations are to be referred to nerve mechanism, we are igno- 

 rant of the course the nerve impulses take, for any rhythmical 

 sympathy existing between the respiratory and vasomotor nerve 



