l66 RESPIRATION 



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 increases 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 de- 

 layed 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 during 

 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 inspiration 

 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 inspira- 

 tion, or forced expiration, the results may be modified, or even 

 reversed, by circumstances not necessary to mention. 



Nervous Mechanism of Respiration. Although the muscles 

 of respiration are of the striated variety, it is by no effort of the 

 will that the movements are kept up. They belong to the class 

 known as automatic; that is, they are, up to certain limits, under 

 the control of the will, but recur in a regular, coordinate and 

 orderly manner without the active intervention of volition. 

 Respiration represents the activity of a self-governing apparatus. 

 These movements constitute a finely coordinated set of contrac- 

 tions contractions which are regulated by means of afferent 

 and efferent nerves under the supervision of the respiratory 

 center. 



