572 CIRCULATION OF BLOOD AND LYMPH. 



started again promptly by adding calcium chlorid in right amounts 

 to the oxalated blood. Regarding the specific part taken by each 

 of the cations in the production of the alternate contractions and 

 relaxations, much diversity of opinion exists, owing to our ignorance 

 of the chemical changes going on in the heart during systole and 

 diastole and to the difficulty of controlling experimental conditions. 

 Thus, while it is an easy matter to control accurately the com- 

 position of the liquids supplied to the heart, a variable and uncon- 

 trollable factor is introduced by the fact that within the tissue 

 elements themselves there is a store of combined calcium, potas- 

 sium, and sodium which may serve to supply these elements to a 

 greater or less extent to the tissue liquids. 



The controversial details upon this question cannot be presented 

 in an elementary book, but the following brief statements may 

 be made regarding one view of the specific effects of the separate 

 cations: (1) The sodium salts in the blood and lymph take the 

 chief part in the maintenance of normal osmotic pressure. The 

 sodium chlorid exists in blood-plasma to the extent of 0.5 to 0.6 

 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 



