142 PHYSIOLOGY. 



rhythmical activity, but that they, in conjunction with sodium 

 chloride, bring about a sustained rhythm. He believes the sodium 

 ion acts by migrating into the muscle-substance and combining with 

 some part of it. Hence, when too many sodium ions have com- 

 bined and taken the place of a number of calcium ions in the 

 muscle, rhythmical beats cease. The poisonous effects of Na ions 

 are antagonized by the addition of a small amount of Ca and K 

 ions. Muscles contract only as long as they contain all three classes 

 of ions (Na, Ca, and K) in a certain proportion, which may vary to 

 a certain extent. 



Numerous substances have been classified on the basis of the 

 degree which they possess of passing through a membrane while in 

 aqueous solution. Those which pass through freely have been 

 found to be capable of crystallization, as a rule, so are termed 

 crystalloids; those which are more tardy in their osmosis through 

 a separating membrane have been ascertained to be noncrystalliz- 

 able, but gluelike in nature, hence are known as colloids. The col- 

 loids are very feeble in all chemical relations, the reverse being true 

 of the crystalloids. Examples of colloids are seen in albumins, 

 gelatin, and starch, while alcohol, sugar, and ordinary saline sub- 

 stances form good examples of crystalloids. 



Osmotic Pressure of Proteids. It is supposed that proteids in 

 solution exert little or no osmotic pressure. The blood contains 

 about 6 per cent, of proteids. Starling, however, believes the pro- 

 teids do exert a small osmotic pressure equal to about 30 millimeters 

 of mercury. By reason of the want of this osmotic power the 

 albumins and globulins remain in the blood. 



Gram-molecular or Mol. Solution. A gram-molecule of any sub- 

 stance is the quantity in grams of that substance equal to its mole- 

 cular weight. A gram-molecular solution of any substance is the 

 quantity of grams of that substance equal to its molecular weight. 

 A gram-molecular solution is one which contains a gram molecule 

 of the substance per liter. Thus, 58.5 grams of sodium chloride 

 (Na = 23.05, Cl = 35.45) in a liter is a gram-molecular solution of 

 sodium chloride. 



Physiological Application. The tissues are composed of cells 

 which are small osmotic sacs, whilst the liquids surrounding them 

 in which the phenomena of metabolism are carried on are always 

 very dilute electrolytes. The ions and water can, as a rule, travel 

 in both directions through the wall or protoplasmic layer of the 

 cell. But not all kinds of substances from the lymph can travel 



