. THE CIRCULATION OF THE BLOOD 307 



According to the first view the stimulus is neurogenic, according to the 

 second view myogenic. 



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

 nounced 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 absence of nerve-cells in portions of the heart-muscle, which never- 

 theless exhibit rhythmic contractions for quite a long period of time; the 

 rhythmic beat of the embryonic heart before the migration of nerve-cells, to its 

 walls shows that the stimulus does not necessarily originate in nerve-cells. 

 Moreover, Porter has conclusively shown that the apex of the dog's heart, 

 which is generally believed to be totally devoid of nerve-cells, can be made 

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

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

 matically, without a cause, it is a fair assumption that the exciting cause of 

 the contraction arises within the muscle-cells themselves, and that it is in 

 all probability the outcome of a reaction between the chemic constituents 

 of the blood or lymph on the one hand, and the chemic constituents of the 

 muscle-cells on the other. The discovery that some of the inorganic salts of 

 the blood have a specific physiologic action on the heart-muscle was made in 

 1882 by Ringer. Since then, many attempts have been made to isolate 

 these constituents, to determine not only their individual, but also their 

 collective action, when combined in proportions approximating those in 

 which they exist in the blood. 



The Action of Inorganic Salts. i. On the Frog and Terrapin Heart. 

 The inorganic salts which are most directly concerned in exciting and sustain- 

 ing the heart-beat are sodium chlorid, calcium phosphate or chlorid, and potas- 

 sium chlorid. A combination of these salts in the proportions in which they 

 exist in the blood was first suggested by Ringer and is made by saturating a 

 0.65 per cent, solution of sodium chlorid with calcium phosphate, and then 

 adding to each 100 c.c., 2 c.c. of a i per cent, solution of potassium chlorid. 

 A frog's heart immersed in this solution will continue to beat for some hours. 

 A combination of the chlorids of sodium, calcium, and potassium in amounts 

 which will vary for different animals is equally efficient in maintaining the 

 heart-beat. 



The collective as well as the individual actions of these salts have been 

 strikingly brought out by the experiments of Profs. Howell and Greene, from 

 whose published results the following statements are derived. Instead of 

 employing the entire heart, they used for various reasons strips from the 

 terminations of the venae cavae and from the ventricle of the terrapin heart. 

 The proportion of the inorganic salts most favorable for the contraction of 

 the vena cava strips is the following: viz., sodium chlorid, 0.7 per cent.; 

 calcium chlorid, 0.026 per cent.; potassium chlorid, 0.03 per cent. When 

 vena cava strips are immersed in this solution, they begin in a short time to 

 exhibit rhythmic contractions which may continue for several days. In the 

 same strength of solution the ventricular strips remain inactive but if the 

 percentage of the calcium chlorid be raised from 0.026 per cent, to 0.04, or 

 0.05 per cent., spontaneous contractions soon develop and continue for 



