256 THE PROPERTIES OF PROTOPLASM 



Tissue. Percentage of water 



Fatty tissue 6-10 



Bone (extremities and skull) , . . . . ... . . . 14-22 



Bone (vertebra and ribs) , ..... ... . . 16-44 



Tendon . . . .'.'. . . . 56-68 



Brain, white substance . . ... . ; . . . .. . . 68-70 



Muscular tissue 75-78 



Thyroid gland ... . ...... . . . . . 77-82 



Thymus ' . . . 81 



Brain, gray substance 82-85 



It will immediately be noted that the percentage of water is highest 

 in those tissues which are undergoing the most rapid metabolic changes 

 and which are called upon to function, in a chemical rather than a 

 structural manner, most rapidly and frequently. The percentage of 

 water is lower in adult than in embryonic -tissue, and decreases with 

 advancing age of the tissues, and diminution of the speed of metab- 

 olism. Living tissues which are exceptionally poor in water or which 

 withstand dessication, such as seeds or bacterial spores, represent life 

 latent, but arrested, only to be resumed in full vigor upon the readmis- 

 sion of water. 



The force which impels the movement of water into or out of the 

 the elements of living matter is the difference between the Osmotic 

 Pressure of the fluids within the cell on the one hand, and the external 

 medium which bathes the cell on the other. The manner in which 

 this force may impel the migration of water will be evident if we 

 consider the mechanism by which it originates. We may suppose 

 that the molecules of a substance in solution are in a state of con- 

 tinuous motion, as, indeed, their diffusibility shows that they must 

 be. Let us consider the condition of affairs in a vessel filled with 

 water (Fig. 9) and divided into two parts by a partition A-B, on the 

 right-hand side of which we introduce such an amount of some diffus- 

 ible substance, such as glucose, that there are ten molecules of glucose 

 in the mixture for every ninety molecules of water. Evidently, on 

 the left-hand side of it every molecule which collides with this parti- 

 tion will be a water-molecule, but on the right-hand side every tenth 

 molecule will be a sugar-molecule. 



If, now, the partition A-B is constructed of such material that it is 

 porous to water, but impermeable for more bulky molecules such as 

 those of sugar, it is evident that out of 100 molecules bombarding the 

 partition from the left all will pass through into the right-hand chamber, 

 while out of 100 molecules bombarding the partition from the right, 

 only 90 will be able to penetrate into the left-hand chamber. In any 

 given interval of time, therefore, an excess of water molecules will have 

 entered into the right-hand chamber, and this excess will be directly 

 proportionate to the concentration of sugar dissolved therein. 



Such a partition as that which we have described constitutes what 

 is known as a Semipermeable Membrane, and membranes having the 

 characteristic of permitting the passage of water but not of dissolved 

 substances are very numerous. The one most frequently employed 



