CHAPTER XXXI. 



THE RATE OF THE HEART BEAT AND ITS VARIA- 

 TIONS UNDER NORMAL CONDITIONS. 



The rate of heart beat changes quickly in response to variations 

 in either the internal or external conditions. Therein lies, in fact, 

 the great value of the regulatory (inhibitory and accelerator) nerves. 

 Through their agency, in large part, the pump of the circulation is 

 reflexly adjusted to suit the changing needs of the organism and 

 adapted more or less successfully to alterations in the external 

 environment. The variations in the rate of beat may be considered 

 under three general heads: (I) Fixed adjustments to the different 

 mechanical conditions of the circulation. (II) Variations caused 

 by reflex effects upon the inhibitory or accelerator nerves. (Ill) 

 Variations caused by changes in the physical or chemical conditions 

 of the blood. 



The Fixed Adjustments of Rate. When we speak of the 

 normal pulse rate we mean the rate in an adult when in a condition 

 of mental and bodily repose. Examination shows that under these 

 circumstances there are great individual variations. The average 

 normal rate for man may be estimated at 70 beats per minute; 

 for woman, 78 to 80 beats; but the normal rate for some individuals 

 may be much lower (50) or much higher (90). Among the condi- 

 tions for which the heart rate shows a certain constant fixed adapta- 

 tion the following may be mentioned: 



Variations with Sex. The average pulse rate in women is, as 

 a rule, higher than that in men, and this difference seems to hold 

 for all periods of life. 



Variations with Size. Tall individuals have a slower pulse 

 rate than short persons of the same age. Several observers have 

 thought that they could detect a constant relationship between 

 size and pulse rate. Thus, Volkmann believed that the pulse 

 rate varies inversely as the five-ninth power of the height. In 

 the same direction it is found that small animals, as a rule, have 

 a higher pulse rate than larger ones. Thus, elephant, 25-28; 

 horse and ox, 36-50; sheep, 60-80; dog, 100-120; rabbit, 150; 

 small rodents, 175 or more. It would seem, from these facts, that 

 the fast rate in the small animals, with their shorter circulatory 

 path and smaller volume of blood, is necessary to the mechanical 



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