CHAPTEE IX 



PHYSIOLOGY OF CAKDIAC MUSCLE AND NERVES 



CONTENTS. 1. Intrinsic processes by which cardiac rhythm is determined and 

 regulated. 2. Extrinsic chemical conditions of cardiac activity. 3. Effects ot 

 ligation and section on different parts of the heart. 4. Automatic or reflex 

 activity of heart. 5. Myogenic or neurogenic origin of cardiac rhythm. 6. 

 Evidence for these conflicting theories. 7. Special mode in which cardiac muscle 

 reacts to external stimuli. 8. Regulation of cardiac rhythm by nervous system : 

 inhibitory or diastolic nerves. 9. Accelerator or systolic nerves. 10. Theory of 

 anabolic action of diastolic nerves and katabolic action of systolic nerves. 11. 

 Afferent nerves of heart or other parts of the body which influence cardiac rhythm. 

 12. Nerve-centres for cardiac nerves ; their tonic excitability, and theory of 

 regulation of cardiac rhythm. Bibliography. 



THE mechanical functions of the heart and vessels, as discussed in 

 the last two chapters, are modified by a variety of oscillations or 

 changes. Between certain limits these changes not only come 

 within the scope of normal vital activity, but also satisfy the 

 physiological necessity for adapting both the general velocity of 

 circulation, and the distribution of blood in the several parts of the 

 body, to the different external conditions and temporary needs of 

 the whole organism, or of its several organs or tissues. 



The physiological changes in the activity of the heart consist 

 in increased or diminished frequency or force of its beats ; the 

 physiological changes in the activity of the vessels consist in their 

 dilatation or constriction (particularly in the small arteries), as 

 determined by the expansion or contraction of the muscle cells of 

 which they are constructed. 



Even to the lay mind, it is obvious that these modifications 

 and adaptations of the activity of the heart and vessels depend 

 essentially on the nervous system, which is the supreme regulator 

 of all important vital functions. From the fact that the heart 

 responds to the psychical emotions by various modifications of its 

 activity, sprang the old Aristotelian belief that it is the seat of the 

 soul a belief still surviving in popular ideas and expressions. The 

 fact that the emotions readily produce blushing and pallor of the 

 countenance shows that the nervous system is capable, independent 

 of the circulatory centre, of modifying the blood supply to the 



several vascular regions. 



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