PHYSICAL PKOCESSES IN CELLS. 45 



a collection of capillary tubes. When such a solid (e.g., a piece of chalk) 

 is immersed in a fluid which is capable of wetting it, the fluid will enter 

 into the pores of the solid through capillarity, and will remain even after 

 the body is removed from the fluid. The solid is then said to have 

 absorbed fluid by imbibition. Organic bodies also are capable of absorb- 

 ing fluid by imbibition, but the process is somewhat different from that 

 of the inorganic porous body. 



Every organic bod}^, no matter what its consistency, contains always 

 a large amount of water in its composition, to which fluid, as we shall 

 find later, many of the physical properties of the tissues are due. When 

 inorganic bodies contain fluid, that fluid is held in one of two ways, 

 either chemically united with the body, as in hydrates, or as water of 

 crystallization ; or mechanically in the pores of the solid. Organic bodies 

 occupy a mean between these two. The water in their composition is 

 not in a form of chemical combination, nor is it held mechanicalty in 

 pores, as in the porous inorganic body, though the conditions are some- 

 what similar to the latter case. That there is a difference, however, is 

 proved by the different effects of the abstraction of water from organic 

 and porous inorganic bodies. The physical characters of porous, inor- 

 ganic solids, such as baked clay, are not seriously altered by the removal 

 of water contained in their pores. The abstraction of water from semi- 

 solid organic bodies, on the other hand, entirely changes their physical 

 and physiological properties. A piece of connective tissue in its fresh 

 condition is soft, white and glistening, flexible, extensible and elastic. 

 When the water which it contains is removed by drying, it shrivels up, 

 becomes rigid, yellowish in color, brittle, and loses weight. If it be then 

 immersed in water its previous characters will be restored. This differ- 

 ence in the manner in which the water entering into the composition of 

 organic and inorganic bodies is held is explained by the assumption that 

 in the porous inorganic body the water occupies comparatively large 

 spaces between particles of solid, while in the organic body the water 

 surrounds the ultimate molecules of the body. Organic tissues, ma}' 

 therefore be defined as bodies whose intermolecular spaces are filled 

 with fluid. 



As the fluids are held in a different manner in inorganic and organic 

 bodies, it is natural to find that the way in which the fluid is absorbed 

 differs in the two cases. 



When a dry, porous, inorganic body, such as a piece of chalk, is 

 thrown into w r ater, the water enters the pores of the chalk by capillarity 

 and displaces the air which was contained in its pores. It increases in 

 weight by the addition of the weight of the absorbed water, but does 

 not increase in volume. When a dry organic body, such as a piece of 

 gelatin, is thrown into water no air is displaced, and yet many times 



