No. 599] OSMOTIC PRESSURE OE THE BLOOD 6 13 



pressure of 22.4 atmospheres. A gram molecule of any 

 other gas under the same conditions has the same pres- 

 sure. Van't Hoff in his theory of solutions established 

 the fact that a substance in solution behaves as a gas oc- 

 cupying the same volume as the solution and the laws 

 which solutions obey are analogous to those which are 

 followed by gases. Therefore a gram-mol of a substance 

 dissolved in a liter of pure water would have the same 

 pressure as a gram-mol of gas, i. e., 22.4 atmospheres. 

 This pressure property of dissolved substances is called 

 osmotic pressure. Since the blood and hotly fluids con 

 tain salts and other substances in solution, these fluids 

 therefore have a certain osmotic pressure. It is well 

 known that a salt solution has a lower freezing point than 

 that of pure (distilled) water. The difference is propor- 

 tional to the difference in concentration. Since the os- 

 motic pressure depends on the concentration, it follows 

 that the amount of the depression of the freezing point of 

 the solution below that of distilled water is a measure of 

 the osmotic pressure. The osmotic pressure stated in 

 atmospheres can be readily obtained from the "A" or 

 depression of the freezing point by the use of the fol- 

 lowing formula. Osmotic pressure in atmospheres = 

 (A X 22.4)/1.85. 



The blood of a vertebrate serves two double purposes. 

 It carries oxygen to tissues and carbon dioxide away. 

 This is its respiratory function. It also carries nutrients 

 to tissues and wastes of metabolism from tissues. We 

 might call this the nutrient function. But the blood of 

 the earthworm is mainly a respiratory fluid. The body 

 cavity is filled with foods absorbed directly from the in- 

 testine and distributed by the peristaltic movements ot 

 the body to the various tissues. In insects the air is car- 

 ried directly to the tissues through trachea; while a so- 

 called heart lying on the dorsal side of the mtesti 

 open at its anterior and posterior ends 

 and distribution of food absorbed into the body mvi 

 from the intestine. The indefiniteness of the ter 

 ' ' blood " is at once apparent. Most persons in using th 



churning 



