THE RATE OF THE HEART BEAT. 547 



fulfillment of the functions of the blood, and has been preserved 

 by natural selection. 



Variations with Age. In line with the last condition it is found 

 in man that the pulse rate is highest in infancy, sinks quite rapidly 

 at first and then more slowly up to adult life, and rises again 

 slightly in very old age at the time that the body undergoes a 

 perceptible shrinkage. The most extensive data upon this point 

 are found in the works of the older observers.* According to Guy, 

 a condensed summary of the average results obtained at different 

 periods of life, both sexes included, may be given as follows : 



At birth 140 



Infancy 120 



Childhood 100 



Youth 90 



Adult age 75 



Old age 70 



Extreme age 75-80 



The Variations in Pulse Rate Effected through the In- 

 hibitory and Accelerator Nerves. Most of the sudden adaptive 

 changes of the heart rate come under this head. In the laboratory 

 we find that stimulation of all sensory nerve trunks may affect 

 the heart rate, in some cases increasing it, in others the reverse. 

 In life we find that the pulse rate is very responsive to our changing 

 sensations and especially to mental conditions that indicate 

 deep interest or emotional excitement. In a previous paragraph 

 (p. 543) the physiological cause of this effect has been discussed 

 briefly. It may arise either from a reflex excitation of the accelera- 

 tor nerves or a reflex inhibition of the tonic activity of the inhibitory 

 nerves. The facts at present seem to favor this latter explanation. 

 In addition to these reflexes associated with conscious states the 

 heart is susceptible to reflex influences of a totally unconscious 

 character connected with the states of activity of the visceral organs. 

 For example, after meals the heart beat increases usually in rate 

 and especially in force of beat, thereby counteracting the effect on 

 blood pressure of the large vascular dilatation in the intestinal area. 



Variations in Heart Rate with the Condition of Blood-pressure. 

 It has long been known that when the blood-pressure in the arteries 

 falls the pulse rate increases and when it rises the pulse rate de- 

 creases. Thus, the low blood-pressure that is characteristic of 

 the condition of surgical shock is associated with a very rapid 

 rate of heart beat. There is a certain inverse relationship between 

 pressure and rate which has the characteristics of a purposeful 

 adaptation. The quicker pulse rate following upon the low pressure 

 tends to increase the output of blood and raise the pressure. There 



*See Volkmann, "Die Hamodynamik," p. 427, 1850; also Guy, article 

 " Pulse" in Todd's "Cyclopaedia of Anatomy and Physiology," 1847-49. 



