1 66 RESPIRATION 



which will not completely occupy the increase of space in the 

 dilated thoracic aorta, and there will be a backward "suc- 

 tion" made upon the contents of the arterial tree with a 

 consequent decrease in pressure; but a condition just oppo- 

 site to this seems to obtain. 



During expiration contrary conditions in general are op- 

 erative with contrary results. The intrapulmonary vessels 

 and heart are compressed, but the veins and capillaries more 

 than the aorta, with the result that less blood reaches the 

 heart than during inspiration, and the thoracic aorta being, 

 relatively to the thoracic venae cavae, more dilated now than 

 during inspiration can easily accommodate the decreased 

 amount of blood which it receives. Of course expiration in- 

 creases venous pressure in the veins which enter the thorax 

 back as far as the valves. 



The reason the pressure does not rise with the beginning 

 of inspiration is because a short time is consumed in filling 

 the flaccid intrapulmonary veins, and the first increase of 

 blood is delayed for that purpose instead of passing on to 

 the left side of the heart. Similarly, the pressure continues 

 to rise for a short time after expiration has begun because 

 the large veins are being emptied by pressure during this 

 time and their contents are reaching the heart and being 

 forced into the aorta. 



Movements of the diaphragm and abdominal muscles dur- 

 ing respiration also lend themselves to create like changes in 

 arterial pressure, but the main factors are intrathoracic. 



The fact that the cardiac rate is increased during inspira- 

 tion and decreased during expiration may also have to do 

 with the variations in pressure. 



All the foregoing remarks relative to arterial tension are 

 meant to apply to tranquil respiration. During forced in- 

 spiration, or forced expiration, the results may be modified, 

 or even- reversed, by circumstances not necessary to mention. 



Nervous Mechanism of Respiration. Although the mus- 

 cles of respiration are of the striated variety, it is by no ef- 



