286 TEXT-BOOK OF PHYSIOLOGY. 



beginning of the ventricular diastole as the blood surges back against the 

 closed valves. This has been definitely proved by the fact that the sound 

 disappears when the valves are destroyed or held back by hooks introduced 

 into the aorta and pulmonary artery. It is also possible that the vibration 

 of the column of blood produces an additional tone which adds itself to that 

 produced by the valves. 



The Frequency of the Heart-beat. The frequency of the heart-beat 

 varies with a variety of conditions: e.g., age, sex, posture, exercise, etc. 



Age. The most important normal condition which modifies the activity 

 of the heart is age. Thus : 



Before birth, the number of beats a minute averages 140 



During the first year it diminishes to 128 



During the third year it diminishes to 95 



From the eighth to the fourteenth year it averages 84 



In adult males it averages 72 



Sex. The heart-beat is more rapid in females than in males. Thus 

 while the average beat in males is 72, in females it is usually 8 or 10 beats 

 more. 



Posture. Independent of muscle efforts the rate of the beat is influenced 

 by posture. It has been found that when the body is changed from the lying 

 to the sitting and to the standing position, the beat will vary as follows 

 from 66 to 71 to 8 1 on the average. 



Exercise and digestion also temporarily increase the number of beats. 



A rise in blood-pressure from any cause whatever is usually attended by 

 a decrease, while a fall in blood-pressure is attended by an increase in the 

 rate. 



The Blood-supply to the Heart. The nutrition of the heart, its 

 irritability and contractility, the force and frequency of the beat, are depend- 

 ent on and maintained by the introduction of arterialized blood into and 

 the removal of waste products from its tissue. 



In frogs and allied animals the heart muscle is nourished by the blood 

 flowing through its cavities. During the diastole the blood, under the 

 influence of the slight pressure developed, passes from the interior 

 of the heart into a system of irregular passage-ways or channels, which 

 penetrate the heart-wall in all directions and thus comes into direct contact 

 with the heart-cells. With the beginning of the systole the blood is forced 

 out of these channels into the interior of the ventricle, bringing with it the 

 products of tissue metabolism. In mammals the entire inner surface of the 

 heart, as shown by the investigations of Pratt, also presents a series of openings, 

 the foramina of Thebesius, which lead into a similar series of passage-ways 

 penetrating in various directions the heart-walls, and there are reasons for 

 believing that the heart of the mammal may be to some extent nourished in 

 a manner similar to the manner by which the frog heart is nourished. Thus, 

 if a glass tube be inserted and fastened into the aortic opening of the excised 

 heart of a cat and the interior of the ventricle filled with warm defibrinated 

 blood of the same animal, under a pressure of about 75 mm., the heart will 

 recommence and continue to beat for a period varying from one to several 

 hours, thus showing that the mammalian heart may to some extent so 

 receive nutritive material. By reason of the fact that the metabolism of the 

 heart of the mammal is so much more active than that of the heart of the 



