ABSORPTION FROM THE ALIMENTARY CANAL. 331 



though running in various directions and varying in number and arrange- 

 ment in different animals, take on the whole a longitudinal direction 

 parallel to the long axis of the villus. It has been supposed that in con- 

 tracting and shortening the villus they compress the lacteal and thus empty 

 it, and that when they relax and the villus elongates again, the emptied 

 chamber fills once more. But a different interpretation of their action has 

 been offered somewhat as follows. When the muscular fibres contract they 

 shorten the villus. In thus becoming shorter the body of the villus becomes 

 proportionately broader, since probably no great change of bulk in the 

 reticulum takes place ; in this broadening the part to give way will be the 

 lacteal chamber, which thus becomes broader and larger. When the mus- 

 cular fibres relax, the reticulum, the bars of which have been put on the 

 stretch in a lateral direction, by elastic reaction brings back the villus to its 

 former length, and the lacteal chamber elongates and narrows. On this 

 view the muscular contraction expands and so fills, while the relaxation 

 narrows and so empties, the lacteal chamber. Whichever view we adopt, we 

 may at least conclude that contractions and relaxations of the muscular 

 fibres in some way or other alternately fill and empty the lacteal chamber, 

 and in all probability, 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. 



265. The absorption of diffusible substances and of water. On the pro- 

 visional 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 bloodvessels and not into the 

 lacteals. 



The network of capillary bloodvessels is spread 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 capillaires, 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. 255) 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 epithelial cells, 

 through the attenuated reticular lymph-space between the basement mem- 

 brane and the capillary wall, and through the capillary wall into the blood 

 current. Their passage consists of two stages : that through the epithelial 

 cells from the intestine to the lymph-space, and that from the lymph-space 

 into the bloodvessels. 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 



