This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Frequency Of The Heart's Pulsations varies in health with different circumstances, but principally according to age. In infancy the beats are 120 or more per minute; in early life they quickly diminish in frequency; in the adult they are about 75 in the male, and 85 in the female; and in old age the average is lower.
The heart's action is easily influenced in its regularity, strength, and rapidity by the amount of blood in the body, and by the nervous impressions conveyed from other parts. Thus, it may be weak from want of blood, or, when the deficiency is sudden or great, it may be fluttering or irregular; and, on the other hand, the rarer phenomenon is occasionally observed of interference with the heart's action dependent on a superabundance of blood. Emotions also, and conditions of the viscera, send impressions through the nerves which readily disturb the heart. But it is important to observe that the rhythmic action may continue when all connection with other parts has been cut off. A turtle's or a frog's heart will continue to beat when removed from the body, and the successive contractions of its parts will continue to take place in regular sequence, even though there is no longer any blood to stimulate it. When it is divided vertically the portions continue to beat, and when divided transversely the rhythm continues in the basal part, but is lost in the apex. There are not only numerous nerves, but likewise minute nerve-centres, the ganglia of Remak, scattered over the heart; and by these, kept in communication with one another by the copious nerves in the auriculo-ventricular groove, the action is immediately governed (p. 216). No doubt it is difficult to understand how the nerves are stimulated to produce rhythmic contraction; but it must not be forgotten that the pulsation of the heart is only one of a great number of instances of periodicity in nervous action, and that any nervous action frequently repeated has a tendency to go on recurring.