420 ABSORPTION OF DIFFUSIBLE SUBSTANCES. [BOOK n. 



different animals, take on the whole a longitudinal direction 

 parallel to the long axis of the villus. It has been supposed 

 that in contracting 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 muscular fibres relax, 

 the reticulum, the bars of which have been put 011 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 con- 

 tractions 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 nar- 

 rower ; when the muscular fibres relax and the villus elongates, 

 the columnar cells return to their previous form. The alter- 

 nating 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. 



253. 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 pro- 

 visional assumption that these pass into the blood vessels and 

 not into the lacteals. 



The network of capillary blood vessels 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 intes- 

 tine are filled and distended, so that at a time when absorption 

 is taking place these meshes between the capillaries are unusu- 

 ally narrow. From the interior of these capillaries, here as 

 elsewhere, transudation is taking place ; these capillaries sup- 

 ply the lymph which helps to fill up the labyrinth of the retic- 

 ulum and the lacteal chamber. But to a much greater extent 



