264 PHYSIOLOGY. 



All of the sensory nerves of the body have a reflex relation to the 

 pneumogastrics. Even pinching the skin of some fishes is sufficient to 

 stop the heart. Irritation of the branches of the fifth nerve in the 

 rabbit by ether and other vapors can stop the heart. There are 

 reasons to believe similar results can occasionally be obtained in man. 



SWALLOWING FLUIDS. Experimenters have demonstrated that 

 swallowing interferes with or even may abolish for a short time the 

 cardio-inhibitory action of the vagus. By reason of this the pulse- 

 rate is greatly increased. Sipping a wineglassful of water will raise 

 the pulse-count 30 per cent. In this way water can be made to behave 

 as a powerful cardiac excitant. The course of the impulse is along 



Fig. 81. Irritation of Nervus Depressor in a Rabbit, Causing a Fall 

 of Arterial Tension. (GLEY.) 



PC, Carotid pressure. E, Time of irritation of nerve by the induced current. 

 T, Time recorded every 2 seconds. 



afferent fibers of the nerves supplying the oesophagus to the cardio- 

 inhibitory center, whose tonus is reduced. 



Pathological Action. In disease we often have a slow pulse from 

 inflammatory affections of the peritoneum. We can distinguish 

 better if a slow pulse is due to increase of the inhibitory power of the 

 vagus by giving a dose of atropin, which will accelerate the pulse. If 

 the slowing is due to weak heart-muscles, as after pneumonia or 

 typhoid fever, the atropin will be without effect. The vagus center can 

 also be unduly excited by depressing emotions. As afferent impres- 

 sions can either excite or inhibit the vagus center, we can have from 

 sensory irritation a rapid pulse, as a soldier suddenly startled by an 

 explosion of a gun. Vagus acceleration can never be more than 150. 



