520 ABSORPTION OF DIFFUSIBLE SUBSTANCES. [BOOK n. 



and relaxations of the muscular fibres in some way or other 

 alternately fill and empty the lacteal chamber, and in all probabi- 

 lity, at all events during digestion, rhythmical contractions of these 

 fibres are continually going on. When the villus is shortened by 

 the contraction of the muscular fibres, the columnar cells are 

 compressed, becoming longer and narrower; when the muscular 

 fibres relax and the villus elongates, the columnar cells return to 

 their previous form. The alternating changes of form to which 

 the columnar cells are thus subjected, and the alternating changes 

 of pressure taking place in the reticulum, may also serve to promote 

 the passage of material through the one and through the other. 



312. The Absorption of Diffusible Substances and of Water. 

 On the provisional assumption which we have made that the 

 proteids are converted into peptone, we may consider, for the 

 present at all events, peptone, sugar and soluble salts as together 

 forming a class distinguished from fats by their being diffusible, 

 some more so than others. And we have made the further 

 provisional assumption that these pass into the blood vessels and 

 not into the lacteals. 



The network of capillary blood vessels is spread as we have 

 seen ( 262) immediately beneath the basement membrane, and 

 all the material which enters the lacteal chamber has to run 

 the gauntlet of the meshes of this network. During digestion the 

 capillaries of the intestine are filled and distended, so that at a 

 time when absorption is taking place these meshes between the 

 capillaries are unusually narrow. From the interior of these 

 capillaries, here as elsewhere, transudation is taking place ; these 

 capillaries supply the lymph which helps to fill up the labyrinth 

 of the reticulum and the lacteal chamber. But to a much greater 

 extent than elsewhere (cf. 302) this current of transudation from 

 within the capillary to without is accompanied by a reverse current 

 from without to within. The diffusible substances in question 

 pass from the intestine through the layer of epithelium cells, 

 through the attenuated reticular lymph-space between the base- 

 ment membrane and the capillary wall, and through the capillary 

 wall into the blood current. Their passage consists of two stages ; 

 that through the epithelium cells from the intestine to the lymph- 

 space, and that from the lymph-space into the blood vessels. These 

 two stages may be expected to differ, seeing that the structures 

 concerned are different; but we may at first consider them as 

 one, and speak of the passage from the intestine into the blood as 

 a single event. 



In speaking of these substances as diffusible we are using the 

 term in reference to the well-known passage of such substances 

 through thin membranes or porous partitions. When a strong 

 solution of sugar or of common salt is separated by a thin mem- 

 brane (vegetable parchment, dead urinary bladder, dead intestine, 

 &c.) from a weak solution of sugar or of salt, the sugar or salt 



