2 9 6 HUMAN PHYSIOLOGY 



inspiration, but is really due to changes in the cardio-motor 

 centre the accelerating part of which has its activity in- 

 creased during inspiration, while the inhibitory part is more 

 active during expiration. This is thus a reflex effect from 

 the lung through the vagus, and it may be in part due to the 

 proximity of the centres in the medulla. 



But not only is the pulse more rapid during inspiration 

 and slower during expiration, but the waves are smaller 

 during inspiration and larger during expiration. The size 

 of the wave depends much upon the pressure of blood in the 

 arteries, and this change in the pulse thus leads to the fuller 

 consideration of the changes in the arterial pressure due to 

 respiration. 



2nd. Changes in Blood Pressure. If a tracing of the 

 arterial pressure and of the respiratory movements are taken 

 at the same time, it is found that there is a general rise of 

 pressure during inspiration and a general fall during expira- 

 tion, but that at the beginning of inspiration the pressure is 

 still falling, and at the beginning of expiration it is still 

 rising (Fig. 120, p. 256). This influence of respiration on 

 arterial pressure is chiefly a mechanical one, depending on 

 the variations in the pressure in the thorax during inspiration 

 and expiration. 



During inspiration the pressure in the thorax falls to 

 below the atmospheric pressure, and thus during this period 

 the heart and great vessels are under a diminished pressure. 

 This diminution in pressure has little influence on the thick- 

 walled ventricles and arteries, but tells markedly on the thin- 

 walled auricles and veins. In these there occurs a diminution 

 in pressure, which, in the case of the vena cava, may fall 

 below an atmospheric pressure, and as a result an increased 

 flow of blood into these vessels from the veins outside the 

 thorax takes place (Fig. 129, p. 275). 



But when more blood enters the heart the activity of the 

 organ is increased, and more blood is pumped through it into 

 the arteries, and the pressure in these rises. This explains 

 the great rise in arterial pressure during inspiration. 



During expiration the pressure in the thorax rises to above 

 the atmospheric pressure, and thus the pressure on the vessels 

 in the thorax is increased. This tells on the thin-walled 



