136 HUMAN PHYSIOLOGY. 



The fundamental condition for the continuance of the beat is 

 the maintenance of the irritability. So long as this persists the 

 heart will respond to its appropriate stimulus. The irritability of 

 the heart within the body is dependent on the supply of blood coming 

 through its nutrient vessels or flowing through its cavities. Outside 

 the body, the irritability can be maintained for some hours by similar 

 methods. 



The Nature of the Stimulus. The presence of nerve cells in the 

 walls of the heart, their relation to the muscle cells, the pronounced 

 activity of the sinus of the frog heart where they are very abundant ; 

 the feeble activity of the apex where they are absent gave rise to the 

 idea that the stimulus is a nerve impulse rhythmically and auto- 

 matically discharged by these nerve cells. This view is no longer 

 entertained. It has been demonstrated that portions of the heart 

 muscle, that do not contain nerve cells, will continue to exhibit 

 rhythmic contraction for some hours if supplied with oxygenated and 

 defibrinated blood ; that the embryonic heart contracts rhythmically 

 before nerve cells have migrated to its walls. 



The stimulus therefore evidently arises within the heart muscle. 

 In other words, it is my o genie and not neurogenic in origin. The 

 stimulus is now believed to be chemic in character and due to a 

 reaction between the inorganic salts in the muscle cells and those in 

 lymph by which they are surrounded. 



The Influence of the Central Nerve System on the Action of 

 the Heart. Though the heart beat is independent of the central 

 nerve system, it is to a considerable extent modified by it either 

 in the way of inhibition or augmentation. In all classes of animals 

 the heart not only contains localized collections of nerve cells, but 

 is also connected with the central nerve system by two sets of 

 nerve fibers. 



In the frog heart a group of nerve cells is found in the sinus at 

 its junction with the auricle, and known as the crescent or ganglion 

 of Remak ; a second group is found at the base of the ventricle on 

 its anterior aspect and known as the ganglion of Bidder ; a third 

 group is found in the auricular septum, known as the septal ganglion, 

 or the ganglion of Ludwig. These cells were formerly regarded 

 as the source of the stimuli for the heart's contraction. They are 

 regarded now as trophic in function and influencing in some way 

 the nutrition of the heart muscle. 



