228 THE CIRCULATION OP THE BLOOD 



tions of the fifth nerve in the mucosa of the upper respiratory passages, 

 as by inhaling ammonia vapor; by stimulation of the sensory nerve end- 

 ings in the pharynx, as by swallowing ; and of the mucosa of the larynx, 

 as when a substance is "swallowed the wrong way." The sensory 

 nerves of the abdominal viscera seem to be particularly active on the 

 vagus center, as is seen in irritation of the sensory nerves of the stom- 

 ach such as occurs in gastritis. Profound inhibition may also be caused 

 by violent stimulation of the mesentery, as from a blow on the abdo- 

 men, or by irritation of the sensory nerves of the intestine, either me- 

 chanical or because of disease. Another interesting illustration of affer- 

 ent vagus stimulation is obtained by pressure on the outer canthus of 

 the eye. This oculomotor vagus reflex, as it is called, is very marked 

 in some individuals. 



Through which of these afferent paths it may be that the constant 

 stimuli are transmitted to the vagus center to enable it to maintain its 

 tone, can not be said, although it is very likely to be through the vis- 

 ceral nerves. 



In considering the cause for an observed change in heart rate, we 

 must of course bear in mind the possibility that the action may have 

 occurred, not through the vagus center, but through the sympathetic 

 center. Thus, when the heart becomes quicker, it may be owing either 

 to diminution in the vagus tone or to an increase in the discharges 

 along the sympathetic nerve from the augmentor center. That such 

 reflex action through the augmentor center does occur under experi- 

 mental conditions has been clearly shown; for example, if both vagus 

 nerves are cut and the peripheral end of one of them stimulated mod- 

 erately, so as to hold the heart at about its normal rate, the stimulation 

 of certain sensory nerves may cause increase in the heart rate. Reflex 

 sympathetic control of the heartbeat is however no doubt much less 

 important than control through the vagus center. When it does exist 

 it means that the actual rate of the heartbeat at any given moment 

 must represent the algebraic sum of two opposing influences, with that 

 of the vagus preponderating. The advantage of such a double inner- 

 vation is that it insures prompter adjustment of the beat. If, for ex- 

 ample, for any reason quickening of the heart rate is necessary, it is 

 brought about most promptly if the vagus tone is diminished at the same 

 moment that the sympathetic tone is increased. Such reciprocal action 

 of antagonistic influences is the usual rule in the animal economy. Thus, 

 when the knee joint flexes, it does so not merely because stimulating 

 impulses are transmitted to the hamstring muscles, but also because at 

 the same moment inhibiting impulses are transmitted to the extensor 

 muscles (see page 915). 



