10 MOTION OF THE JUICES OF THE ANIMAL BODY. 



lustre. The fibrine and the cartilages of the ear, which desiccation had rendered 

 horny and transparent, again become milk-white and elastic. 



The power which the solids of the animal body possess of taking up water 

 into their substance, and of being penetrable to water, extends to all fluids allied to 

 water, that is miscible with it.* In the dried state, the animal solids take up fluids 

 of the most diverse natures, such as fatty and volatile oils, ether, bisulphuret of 

 carbon, &c. This permeability to fluids is possessed by animal tissues in common 

 with all porous bodies ; and no doubt can be entertained, that this property is 

 determined by the same cause which produces the ascent of fluids in narrow tubes, 

 or in the pores of a sponge ; phenomena, which we are accustomed to include 

 under the name of capillary action. 



One condition, essential to the permeability of porous bodies for fluids (or their 

 power of absorption), is their capability of being moistened ; or the attraction 

 which the particles of the fluid and the walls of the pores or tubes have towards 

 each other.t A second condition is the attraction which one particle of the fluid 

 has to another. We have no means of estimating the absolute size of the particles 

 or molecules of a fluid, such as water, but they are certainly infinitely smaller than 

 the measurable diameter of a tube, or of the pores of a porous body. It is obvious, 

 therefore, that in the interior of a capillary tube or pore, filled with a fluid, only a 

 certain number of the fluid molecules are in contact with the walls of the tube, and 

 attracted by them ; while in the middle of the tube, and from thence towards its 

 parietes, fluid molecules must exist which only retain their place in virtue of the 

 attraction which the molecules, attracted .by the parietes, exert on those not so 

 attracted ; that is, by the cohesive attraction of the fluid. 



Liquids flow out of capillary tubes, which are filled with them, only when some 

 other force or cause acts, because capillary attraction cannot produce motion 

 beyond the limits of the solid body which determines the capillary action. 



The penetration of a fluid into the pores of a porous body, is the result of 

 capillary attraction ; its expulsion can be affected by a mechanical pressure ; and 

 may be accelerated by increasing this pressure, and by all such causes as diminish 

 the mutual attraction of the fluid molecules, or the attraction of the walls of the 

 pores for those molecules. The condition most favorable to the passage of a 

 fluid through the pores of a porous substance under pressure, is when one fluid 

 molecule can be displaced so as to glide away over another. 



The slightest pressure suffices to expel the displaceable particles of water from 

 a sponge ; a higher pressure is required to express the same fluid from bibulous 

 paper ; and a pressure much higher still is necessary in order to cause water to flow 

 out of moist wood.^; We may form some idea of the force with which porous organic 

 substances, such as dry wood, absorb and retain water, if we remember, that by 

 inserting of wedges of dry wood in proper cuts, and subsequently moistening them, 

 rocks may be split and fractured. 



When we compare with the properties just enumerated, which belong to all 

 porous bodies, those properties which are observed in animal substances under the 

 same circumstances, it appears plainly that these animal substances have pores in 

 certain directions ; although these openings are so minute that they are not, in the 

 case of most tissues, perceptible, even with the aid of the best microscopes. 



It has been mentioned that tendons, ligaments, cartilages, &c., contain, in the fresh 

 state, a certain amount of water, which, according to all experiments made on the 

 subject, is invariable ; and that several of their properties depend on the presence 

 of this water. || (CHEVREUL.) When these substances, wrapped in bibulous paper, 

 are subjected to a powerful pressure, a certain proportion of this water is expelled. 

 Fresh and flexible vessels lose, in this way, 37.6 per cent., and the yellow liga- 

 ments of the vertebrae lose 35 per cent, of water. This property, namely, that of 

 losing water under pressure, is only found in porous substances. It is obvious that 

 by pressure, that is, by diminution of the size of the pores, only that portion of 



The tissues absorb other fluids. fThe moistening of porous bodies. 



{Prodigious force with which porous bodies absorb water. 



i Animal tissues are porous. 



lAmount of water expelled by pressure from tissues. 



