PROPERTIES OF THE HEART MUSCLE. 503 



per cent., and the normal osmotic pressure of the blood is mainly 

 dependent upon it. A solution of sodium chlorid of 0.7 to 0.9 per 

 cent, forms what is known as physiological saline, and although 

 not adequate to maintain the normal composition and properties 

 of the tissues it fulfills this purpose more perfectly than the solution 

 of any other single substance. The sodium ions have in addition 

 a specific influence upon the state of the heart tissue. Contractility 

 and irritability disappear when they are absent ; when present alone 

 in physiological concentration in the medium bathing the heart mus- 

 cles they produce relaxation of the muscle tissue. ' (2) The calcium 

 ions are present in relatively very small quantities in the blood, but 

 they also are absolutely necessary to contractility and irritability. 

 When present in quantities above normal or when in a propor- 

 tional excess over the sodium or potassium ions they cause a con- 

 dition of tonic contraction that has been designated as calcium 

 rigor. (3) The potassium ions are present also in very small quan- 

 tities, and, unlike the calcium and sodium ions, their presence in 

 the circulating liquid does not seem to be absolutely necessary to 

 rhythmical activity. Under proper conditions a terrapin's heart 

 beats well for a time upon a solution containing only sodium and 

 calcium salts. The potassium seems to promote relaxation of the 

 muscle and in physiological doses it exercises through this effect 

 a regulating influence upon the rate of beat. When the proportion 

 of potassium ions is increased the heart rate is proportionally 

 slowed, and finally the contractions cease altogether, the heart 

 coming to rest in a state of extreme relaxation, known sometimes 

 as potassium inhibition. (4) It appears from these statements 

 that there is a well-marked antagonism between the effects of the 

 calcium, on the one hand, and the potassium and sodium, on the 

 other. The calcium promotes a state of contraction, the sodium 

 and the potassium a state of relaxation. It is conceivable, there- 

 fore, that the alternate states of contraction and relaxation which 

 characterize the rhythmical action of heart muscle are connected 

 with an interaction of an alternating kind between these ions and 

 the living contractile substance of the heart. 



Connection of the Inorganic Salts with the Causation of the 

 Beat. It is impossible to say positively whether or not the 

 inorganic salts are directly connected with the cause of the beat, 

 that is, with the origination of the inner stimulus. According 

 to one point of view, they are necessary only to the irritability 

 and contractility of the heart tissue. The inner stimulus is pro- 

 duced otherwise by some unknown reaction, but it is not able 

 to cause a contraction of the heart muscle in the absence of the 

 proper inorganic salts. According to another view, the reaction 

 of these ions with the living substance constitutes the inner stimulus, 



