274 THE MECHANICS OF THE HEART 



On account of the larger size of the male body, the heart of the 

 male is less frequent than that of the female, but if a comparison 

 is made between men and women of equal size, no significant differ- 

 ences will be found. The figures ordinarily given for man are: 70 

 beats in the male, 80 in the female, and 90 in children. Even very 

 slight muscular movements increase the rate, while rest decreases 

 it, the lowest value being found after continued quietude in the hori- 

 zontal position. On assuming the erect position the heart beats some- 

 what faster. The figures frequently given are: 75 on lying down, 77 

 on sitting up, and 85 on standing erect. Its frequency is also aug- 

 mented by warm food, or by increasing the temperature of the sur- 

 rounding medium. The same result is obtained if the temperature of 

 the body, as a whole, is raised, as in fever. This augmentation may 

 be shown very clearly by perfusing the heart of a cold-blooded animal 

 with Ringer's solution which it is possible to heat gradually. The 

 force and rate of the heart beat then increase with the temperature 

 until a maximum has been reached at about 30° C. Beyond this point 

 the beats become slower and assume an irregular and fibrillar character 

 until they stop entirely. Very similar tests have been made by N. 

 Martin upon the heart of the cat. This organ ceases to beat at about 

 17° C. and also if the temperature of the perfusing hquid is raised to 

 44° or 45° C. The acceleration obtained during fever may, therefore, 

 be due in large part to the direct action of the blood as it traverses 

 the cardiac chambers. Most generally, the heart of warm-blooded 

 animals beats more quickly and more strongly during the cold seasons 

 of the year, which change is in agreement with the fact that their 

 metabolic activity is greater in winter than in summer. The reduced 

 metaboUsm and heat production coincident with low degrees of tem- 

 perature must be held responsible for the decided decrease in the fre- 

 quency of the heart of hibernating animals. In the bat, for example, 

 a frequency of 28 in a minute during this period gives way to 200 

 per minute during the summer months. Muscular exercise increases 

 the frequency of the heart, because the tissues then undergo more in- 

 tense metabolic changes and require a more copious supply of blood. 

 Decreases in the oxygen content or increases in the carbon dioxid con- 

 tent of the blood increase the rate. 



The Character of the Contraction. — Attention has already been 

 called to the fact that the different segments of the heart do not con- 

 tract simultaneously, but successively, the musculature nearest the 

 venous vestibule being activated first and that nearest the apex last 

 of all. Thus, the contraction of this organ presents several of the 

 characteristics of a peristaltic wave, progressing from its base to its 

 apex. For this reason, it has been said to be similar in character to the 

 curve recorded by skeletal muscle when stimulated with a tetanic 

 current. This fact proves that the cardiac musculature remains in 

 the state of systole for some moments before it again relaxes. Clearly, 

 this peculiarity in the manner of its contraction must tend to produce a 



