300 MANUAL OF PHYSIOLOGY. 



a larger supply of blood during this period. In expiration the 

 negative intrathoracic pressure becomes less negative, the com- 

 pression of the abdominal viscera is relieved, and the flow into 

 the auricle loses somewhat in force. 



It must be carefully borne in mind that the left side of the 

 heart works under different conditions, for the same variations of 

 pressure affect both the pulmonary veins and the left auricle 

 equally, since they are both included in the thoracic cavity, and 

 are both subjected to a slightly varying negative pressure. The 

 aid given to the flow into the right heart by the low intrathoracic 

 pressure is quite absent on the left side ; so that the thoracic 

 movements do not exert any influence on the flow of blood from 

 the pulmonary to the systemic arteries. But while inspiration is 

 taking place the lungs receive a larger supply of blood ; and from 

 the relative amounts of blood in the different organs it is prob- 

 able that this slight excess, having passed the lungs, arrives at the 

 left ventricle at the period of expiration. Thus, during expira- 

 tion 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 arterial 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 following 

 facts: 



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

 piration or inspiration ; 



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

 spiration ; 



(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 tho- 

 racic movements, so that the pressure effects are re- 

 versed. 



