HEART 



395 



Rate of heart per minute in different animals : — 



Many factors modify the rate of the heart, among the 

 most important of which are — 



(1) The Period of Life. — The following table shows the 

 average rate of the heart at different ages : — 



Horse 



New born 



Under 1 

 4 years 



year 



92 to 132 per minute 



50 to 68 



50 to 56 ,, 



(2) The Period of the Day. — The rate is generally lowest 

 in the early morning, and quickest in the evening. 



(3) Temperature of the Body. — The rate varies with the 

 body temperature, in man being increased about ten beats 

 with each degree Fahr. of elevation of temperature. 



(4) The condition of the central nervous system may 

 modify the rate of the heart, any disturbance accompanied 

 by emotional changes either accelerating or retarding it. 



(5) Muscular exercise markedly accelerates the heart 

 {Practical Physiology). 



2. Sequence of Events. — The sequence of events making 

 up the cardiac cycle is simpler in the mammal than in the 



frog- 



(1) The contraction starts in the neighbourhood of the 



sino-auricular node. This is indicated by the fact that Lewis 

 has found that this region is the first to become electro- 

 positive, or " zincy," to the rest of the auricles (p. 212). The 

 contraction spreads out rapidly in all directions, over the 

 auricles and up the mouths of the great veins, as the circle 

 of waves produced by throwing in a stone pass over the 

 surface of the pond. It next seems to pass to the strip 



