122 HUMAN PHYSIOLOGY. 



(the time varying with the species of animal from which it has been obtained) 

 it is evident that the beat is independent of the central nerve system. 



The fundamental condition for the continuance of the beat is the main- 

 tenance 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 main- 

 tained 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 automatically 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 myogenic and not neurogenic in origin. The stimulus is now 

 believed to be chemic in character and due to a reaction between the in- 

 organic salts in the muscle cells and those in lymph by which they are sur- 

 rounded. 



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 acceleration 

 or inhibition. 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. 

 This view is no longer entertained. 



In the dog and the mammalian heart generally, the nerve cells though 

 present are not arranged in such definite groups, but are distributed in the 

 terminations of the venae cavae, pulmonary veins, the walls of the auricles and 

 in the neighborhood of the base of the ventricles. 



