246 PHYSIOLOGICAL PHYSICS. [Chap. xxn. 



clean acid, with a little practice in making the instru- 

 ment, are all that is necessary. 



Capillary action in porous bodies. Imbi- 

 bition. Porous bodies may be considered as bodies 

 traversed in certain directions by capillary tubes. 

 Into the interstices of such porous bodies liquids are 

 capable of entering by capillary attraction. Thus, 

 a porous body plunged into water is permeated by 

 the liquid, which remains after the body has been 

 withdrawn from the mass of liquid. This is called IMBI- 

 BITION. The fluid may be expelled by pressure. It is 

 thus that a sponge takes up a large quantity of water, 

 expelled on squeezing. All animal and vegetable 

 tissues are porous, even though the microscope may 

 not be able to reveal the presence of the interstitial 

 spaces. All the tissues of the animal body are, ac- 

 cordingly, permeated by the fluids of the body. The 

 flexibility and silky lustre of tendons are due to the 

 fluid mechanically retained in the tissue. Let the 

 tissue be exposed till it becomes dry, it will then have 

 lost its lustre and a great degree of its flexibility ; and 

 will have become transparent. It will also be lighter 

 than before by the amount of water it has lost. But 

 let it be immersed in water for a time, much of its lost 

 properties will be restored, and its weight will also 

 have been restored by the amount of water imbibed. 



Yellow elastic tissue, cartilage, the cornea of the 

 eye, give results of a like sort. The organic tissues, 

 such as wood, exhibit similar phenomena. But such 

 tissues placed in water will imbibe, not only their 

 normal quantity of water, but a quantity greatly in ex- 

 cess of it. The increased quantity of fluid will distend 

 the narrow passages in which it is lodged, and will 

 thus increase the bulk of the tissue. But with this 

 distension there is .brought into play elastic reaction, 

 and resistance to the distension arises. The water 

 will continue to be imbibed so long as the capillary 



