108 



SELECTING POWER OF MEMBRANES. 



cause they arc porous ; that the so-called selecting power is purely 

 Fig. 41. physical, as are the separations and apparent decomposi- 



tions to which it gives rise. When a drop of colored 

 water is put upon chalk, the water sinks in, but the color 

 is left on the surface. When weak alcohol is tied up 

 in a bladder, the water will escape through the pores, 

 and the spirit become anhydrous at last. 



If we take a glass tube, <2, a, Fig. 41, over the lower 

 end of which a piece of peritoneum, or other delicate 

 membrane, &, &, is tightly tied, and half fill it with litmus- 

 water, and then place it in a glass of alcohol, c, c, the 

 level of the liquids inside and outside being adjusted ac- 

 cording to their specific gravity, so that there may be no 

 hydrostatic pressure either one way or the other through 

 the pores of the peritoneum as soon as the arrangement 

 is completed, if the observer be so placed as to view it by 

 transmitted light, he will see the water descending from 

 the pores of the peritoneum in stride and streams through 



Selecting power of a r r 



membrane. the alcohol in a perfectly colorless state. Th6 membrane, 

 therefore, has absorbed and transmitted the water, but has refused to 

 the coloring matter a passage. It is to this particular experiment that 

 a^usion was made when speaking of the non-coloration of the chyle 

 when certain coloring material had been mixed with the food. Such 

 illustrations may therefore satisfy us that the selecting power of organic 

 porous textures, like that of inorganic ones, is dependent on simple 

 physical circumstances, and for these reasons I exclude from the mech- 

 anism of animal absorption the influence of any vital or other mysterious 

 principle, and adopt the sentiment of the Abbe Hauy, that "those specious 

 causes and imaginary powers, to which, in the Middle Ages, all natural 

 phenomena, even those of an astronomical kind, were referred, but which, 

 through the genius of Newton and Laplace, have been banished from the 

 celestial spaces, have taken their last refuge in the recesses of organic 

 beings, and from these retreats positive philosophy is preparing to expel 

 them." 



In view of all the preceding facts, I therefore regard absorption by the 

 Summary of blood-vessels as taking place of necessity, because of the po- 

 tts nature of rous structure of those tubes ; for, though the pores may be 

 too small to be discerned even by microscopic aid, they are 

 abundantly large enough to permit such a percolation. Whatever ma- 

 terial is existing in the chyme in a state of solution in water and also 

 soluble in the blood, passes through the walls of the vessels, and is moved 

 toward the liver, its percolation being greatly facilitated by the onward 

 motion of the blood, in which liquid it is dissolved as fast as it presents 



