178 STUDIES IN ADVANCED PHYSIOLOGY. 



vessels in the brain and the accumulation of blood in 

 organ.) This may further explain why a person in lying 

 down usually has the rate of his pulse lowered, for in the 

 lying posture the pressure of blood in the brain is, of course, 

 suddenly increased. The fact that such an increase of blood 

 pressure should at once by stimulating the cardio-inhibitory 

 center cause the heart to beat slower, is a very fortunate 

 circumstance, for it reduces the possibility of the rupture of 

 blood vessels and consequent hemorrhages. Second, the 

 center is further stimulated by psychic influences. Strong 

 emotions may affect it. Thus, fainting, which is usually 

 due to a flurry of the emotions, results from a stoppage of 

 the heart caused by the inhibitory action of the vagi. 

 When this stoppage lasts but an instant no serious damage 

 results, but when the inhibition has been so violent as to 

 prevent the heart from resuming its beat for even a little 

 while, death may result for lack of circulation. The third 

 exciting cause is the lack of oxygen. If venous blood only 

 is allowed to run through the center the rate of pulse is 

 soon reduced and finally stops altogether. This is, of 

 course, the explanation of the observed fact that in cases 

 of suffocation the pulse sinks so rapidly and the heart ceases 

 to beat so soon. Fourth, this center may be stimulated by 

 afferent impulses coming from other sensory nerves. A 

 severe blow on the pit of the stomach may cause the heart 

 to stand still for an instant. Here the afferent impulse 

 goes from the sensory nerves of the abdomen to the brain, 

 and there acts in an exciting way on the cardio-inhibitory 

 center. Fifth, in addition to these impulses which affect the 

 center directly there are a number of drugs and poisons 

 which act upon the vagus nerve and so affect the beat of 

 the heart. Thus the poison muscarin stimulates the ends 

 of the vagus nerve in the heart and so may bring the heart 

 to a standstill. The drug atropin lames the vagi and so 

 produces an acceleration in the heart beat, while nicotine 

 seems first to stimulate the nerves and so reduce the pulse, 

 then later to lame the nerves and so by weakening the in- 

 hibitory influence to increase the rate of beat. 



