128 A MANUAL OF PHYSIOLOGY 



the physiological mechanism through which the physical 

 changes are brought about. We have now to see that 

 although the heart is a pump, it is a living pump ; that 

 although the vascular system is an arrangement of tubes, 

 these tubes are alive; anj that both heart and vessels are 

 kept constantly in the most delicate poise and balance by 

 impulses passing from the central nervous system along the 

 nerves. 



In many respects, and notably as regards the influence of 

 nerves on it, we may look upon the heart as an expanded, 

 thickened and rhythmically-contractile bloodvessel, so that 

 an account of its innervation may fitly precede the descrip- 

 tion of vaso-motor action in general. 



The Relation of the Heart to the Nervous System. A very 

 simple experiment is sufficient to prove that the beat of the 

 heart does not depend on its connection with the central 

 nervous system, for an excised frog's heart may, under 

 favourable conditions, of which the most important are a 

 moderately low temperature, the presence of oxygen and the 

 prevention of evaporation, continue to beat for days. The 

 mammalian heart also, after removal from the body, beats 

 for a time, and indeed, if defibrinated blood be artificially 

 circulated through the coronary vessels, for several hours. 

 But although this proves that the heart can beat when 

 separated from the central nervous system, it does not 

 prove that nervous influence is not essential to its action, 

 for in the cardiac substance nervous elements, both cells and 

 fibres, are to be found. 



The Intrinsic Nerves of the Heart. In the heart of the frog 

 numerous nerve-cells are found in the sinus venosus, espe- 

 cially near its junction with the right auricle (Remak's 

 ganglion). A branch from each vagus, or rather from each 

 vago-sympathetic nerve (for in the frog the vagus is joined a 

 little below its exit from the skull by the sympathetic), 

 enters the heart along the superior vena cava (pp. 173, 174). 



Running through the sinus, with whose ganglion cells the true 

 vagus fibres, or some of them, are believed to make physiological 

 junction (p. 141), the nerves pursue their course to the auricular 

 septum. Here they form an intricate plexus, studded with ganglion 

 cells. From the plexus nerve fibres issue in two main bundles, 



