THE CIRCULATION OF THE BLOOD 325 



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 thecardio-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. 



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 Influence of Psychic States. It is a familiar personal experience 

 that emotional states, according to their suddenness or intensity may in- 

 crease or decrease the activity of the heart; thus depressing emotions may 

 diminish the activity almost to the point of complete inhibition, while joyous 

 emotions on the contrary may increase both rate and force to the point at 

 which the action becomes tumultuous and even irregular. The effects hi 

 either case are due to nerve impulses descending from the cerebrum to the 

 cardiac center in the medulla. 



The nerve impulses discharged by cerebral cells during the occurrence 

 of both the depressing and joyous emotions exert their influence in all prob- 

 ability, on the cardio-inhibitor center alone. In the case of depressing emo- 

 tions, the nerve impulses excite or increase the normal degree of activity of 

 this center in consequence of which its inhibitor influence on the heart is 

 increased; in the case of joyous emotions, the nerve impulses inhibit the 

 normal degree of activity of the center in consequence of which its influence 

 on the heart is decreased. This permits of a freer play of the cardio-accel- 

 erator center and as a result there is an increased activity of the heart. 



The Causes of the Variations in the Heart-beat. It has been stated 

 elsewhere in the text (page 290), that the rate of the heart-beat is influenced 

 by age, muscle activity, the position of the body, meals, variations in blood 

 pressure, etc. The manner in which these changes are brought about is 

 not, however, always apparent. In addition to variations that are strictly 

 physiological in character there is abundant evidence that other factors, e.g., 

 the action of peripheral stimuli of a physiologic or pathologic character in 

 various regions of the body, can and do cause reflexly at one time or in one 

 individual an acceleration of a marked character, and at another time or 11 

 another or the same individual an inhibition which may be so pronounced 

 as to almost lead to a complete, though temporary standstill of the heart in 

 diastole The records of clinical medicine contain many instances wh 

 show that ocular, dental, gastric, intestinal, uterine and other organic : d 

 orders, as well as various operative procedures in different regions o 

 body cause now an acceleration, now an inhibition of the heart whic 

 be so marked and pronounced as to give rise to serious apprehensu 



The Depressor Nerve. The vagus trunk also contains afferei 



