316 TEXT-BOOK OF PHYSIOLOGY. 



The first explanation, that acceleration of the heart, the result of a 

 peripherally acting stimulus, is due to a stimulation of the cardio-accelerator 

 center by the arrival of nerve impulses coming through afferent nerves, 

 having been made questionable and improbable by the results of Hunt's 

 experiments, the alternative explanation must be that the acceleration is due 

 to an inhibition of the normal activity of the cardio-inhibitor center, and 

 that inhibition is due to an excitation of the normal activity of the cardio- 

 inhibitor center, and hence there follows the corollary that afferent nerves 

 contain two sets of nerve-fibers which are in physiologic relation with the 

 cardio-inhibitor center, one of which when stimulated peripherally inhibits its 

 activity, the other of which when stimulated excites or augments its activity. 



The extent to which both sets of fibers are present in any one afferent 

 nerve is unknown. In the trigeminus it is believed the excitator fibers 

 preponderate for the reason that peripheral stimulation of this nerve is 

 followed by inhibition of the heart; in the sciatic, it is believed the inhibitor 

 nerves preponderate, for the reason that stimulation of the central end of the 

 divided nerve is followed generally by acceleration of the heart. 



It is probable from the effects which follow gastro-intestinal disorders, 

 that the vagus nerve contains both classes of fibers as represented in Fig. 141, 

 inasmuch as stimuli of a pathologic character in one individual may reflexly 

 excite or increase the activity of the cardio-inhibitor center, to be followed 

 by an inhibition of the heart; and in another individual, may reflexly inhibit 

 the activity of the same center and to such an extent that the cardio-accelerator 

 center may be enabled to increase either the rate or the force or both, of the 

 heart movements. Palpitation of the heart from gastric irritation might 

 thus be explained. 



The Influence of Psychic States. The cardio-inhibitor and the cardio- 

 accelerator centers may be increased in activity also by nerve impulses 

 descending from the cerebrum, the result of emotional states; thus depressing 

 emotions according to their intensity may so increase the activity of the 

 cardio-inhibitor center that the heart's action may not only be retarded but 

 even completely inhibited; joyous emotions, on the contrary, may so increase 

 the activity of the cardio-accelerator center or what is more probable inhibit 

 the activity of the cardio-inhibitor center that the heart's action will be 

 increased in both its rate and force. 



From the results of stimulation of the sympathetic (accelerator) and 

 vagus (inhibitor) nerves under a great variety of conditions it has been 

 established that their respective centers are mutually antagonistic; that the 

 activity of the accelerator center at one moment limits the activity of the 

 inhibitor and at another moment is limited in turn by it; that the rate of the 

 heart-beat at each moment is the resultant of the relative degree of activity 

 of the two centers. 



The Depressor Nerve. The vagus trunk also contains afferent fibers 

 stimulation of which not only brings about a reflex inhibition of the heart, 

 but also a dilatation of the peripheral arteries and a fall of blood-pressure 

 through a depressive influence on the vaso-motor centers. To this nerve 

 the term depressor has been given. A consideration of the physiologic 

 action of this nerve will be found in the section devoted to the nerve mechan- 

 isms concerned in the maintenance of the blood-pressure. 



