THE CIRCULATION OF THE BLOOD. 315 



of atropin, which removes temporarily inhibitor control, is not followed by an 

 increase in the rate of the beat. It develops steadily and reaches a maximum 

 at from the twenty-fifth to the thirtieth year. In advanced years the tone 

 again declines. For these and other reasons it is 1 believed that this center 

 is in a state of tonic activity in many if not all mammals, discharging nerve 

 impulses which exert a regulative influence on the cardiac mechanism in ac- 

 cordance with its needs and especially in reference to the variable resistances 

 offered to the flow of blood which the heart must overcome. 



The Factors which Determine the Activity of the Cardio-Inhibitor 

 Center. The question has also been raised as to whether the tonic activity 

 of this center is maintained by central or peripheral stimuli, i.e., whether it is 

 maintained by causes within itself the result of an interaction between the 

 constituents of the cell substance and those of the surrounding lymph, or 

 whether it is maintained by nerve impulses reflected to it through various 

 afferent or sensor nerves. Though both factors play an important part in 

 the maintenance of its activity, the trend of evidence points to the conclusion 

 that the reflected impulses are by far the more important of the two. This 

 latter supposition is supported by the results of direct experimentation upon 

 sensor nerves in almost any region of the body. Thus stimulation of the 

 dorsal roots of the spinal nerves, the trunks of the cranial sensor nerves, the 

 splanchnic nerves, the pulmonary branches of the vagus, etc., gives rise to a 

 more or less pronounced inhibition of the heart. As a rule, stimulation of 

 the peripheral terminations of these nerves is more effective than stimulation 

 of their trunks, hence an explanation is at hand for the cardiac inhibition 

 which results from sudden distention of the stomach and intestines, or 

 operative procedures in the nose, mouth, and larynx. 



Reflex inhibition of the heart, even to the stage of absolute and permanent 

 standstill, eventuating in the death of the individual is a not infrequent 

 result of peripherally acting causes of a pathologic or operative character. 

 From the results of experimental procedures the inference is drawn that 

 normally, nerve impulses, developed by the action of physiologic causes, 

 are reflected continuously from many peripheral regions of the body, and 

 falling into this center gently stimulate and maintain it in a condition of 

 necessary tonicity or activity. 



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

 elsewhere in the text (page 286), 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 in 

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

 as to lead to a complete standstill in diastole. The records of clinical medi- 

 cine contain many instances which show that gastric, intestinal, uterine and 

 other organic disorders as well as various operative procedures in different 

 regions of the body cause now an acceleration, now an inhibition of the 

 heart. 



