PRESSURE CONDITIONS IN LUNGS AND THORAX. 591 



systemic circulation, and therefore doubts whether small changes in 

 the capacity of the lung vessels can have any distinct effect on the 

 inflow into the left auricle. He thinks that the main factor is the 

 increased flow of blood to the right auricle during inspiration, and 

 that this increased amount is then passed on to the left auricle and 

 ventricle, but that this takes some little time, so that the true effect 

 of inspiration is not felt in the aorta at the very beginning of the 

 act. This delay may vary in different animals and may account 

 for the fact that in some animals there is an apparent inversion of 

 the relations to respiration, the aortic pressure falling throughout 

 inspiration and rising during expiration. 



The increased rate of heart beat during inspiration varies as to 

 its degree in different individuals. It has been shown by Fredericq 

 that this change occurs when the chest is widely opened and the 

 respiratory movements can have no mechanical effect upon the heart. 

 He suggests, therefore, that the accelerated pulse during inspiration 

 is due to an associated activity in the nerve centers of the medulla. 

 When the inspiratory center discharges it affects the neighboring 

 cardio-inhibitory center in the direction of inhibition, lessening its 

 tonic activity and thereby increasing the heart rate. 



In artificial respiration carried out by means of a bellows 

 that is, by expanding the lungs with positive pressure all the 

 conditions of pressure in inspiration and expiration are reversed. 

 During such an inspiration the flow of blood to the right heart, and 

 through the lungs to the left heart, is decreased. Respiratory 

 waves of pressure are present under such conditions, but the rela- 

 tions of rise and fall to the phases of respiration are reversed. 



