286 TEXT-BOOK OF PHYSIOLOGY. 



long as it lasts in either class, the irritability and contractility persist. 

 The passage of defibrinated blood through the vessels of the excised 

 heart of the dog may maintain the duration of the irritability for a 

 period of from one to six hours. 



Whatever the immediate or exciting cause of the heart contraction 

 may be, the fundamental condition for its manifestation is the main- 

 tenance of the irritability. So long as this persists at the normal 

 level the heart-muscle will contract in response to the appropriate 

 stimulus. 



Nature of the Stimulus. As the heart continues to beat after 

 removal from the body, it is evident that the stimulus does not origi- 

 nate in the central nerve system but in the heart itself. Two views 

 have been held as to its origin and nature: 



1. That it originates in the nerve-cells found in various parts of the 



heart-muscle; that it is a nerve impulse rhythmically and auto- 

 matically discharged by these cells and transmitted by their 

 axons to the heart-muscle cells. 



2. That it originates in the muscle-cells themselves ; that it is chemic in 



character and due to a reaction between the inorganic salts in the 

 muscle cells and those in the lymph by which they are surrounded. 



According to the first view the stimulus is neurogenic, according 

 to the second view myogenic, in origin. 



The presence of nerve-cells; their relation to the muscle-cells; the 

 pronounced rhythmic activity of the sinus and auricles in which the 

 nerve-cells are abundant ; the feeble activity of the apex, in which they 

 are wanting, these and other facts lend support to the view that the 

 stimulus originates in the nerve-cells. To them have been attributed 

 the power of automatic activity. 



The fact, however, that portions of the heart-muscle in which 

 nerve-cells have not yet been shown to exist can be made to exhibit 

 rhythmic contractions for even a long period of time shows that 

 the stimulus does not necessarily originate in nerve-cells. Porter 

 has conclusively shown thar\the apex of the dog's heart, which is 

 generally believed to be totally devoid of nerve-cell^can be made to 

 beat for hours by feeding it through its nutrient artery with warm defi- 

 brinated blood. Unless it be assumed that the heart-muscle contracts 

 automatically, without cause, it is evident there must be present in the 

 blood some agent or agents which by individual or collective action 

 act as a stimulus to the muscle-cell. Attempts have been made to 

 isolate these agents, to determine not only their individual, but also 

 their cooperative action, when combined in proportions approximat- 

 ing those in which they exist in the blood\The agents known to be 

 directly concerned in exciting and sustaining the heart-beat are. sodium 

 chlorid, calcium phosphate or chlorid, and potassium chlorid v .\ Rin- 

 ger's combination of these salts is made by saturating a 0.65 per cent. 



