548 ABSORPTION OF DIFFUSIBLE SUBSTANCES. [BOOK 11. 



fibres of the villus supply a special muscular pump for the empty- 

 ing and filling of the lacteal chamber. These fibres and small 

 bundles of fibres though running in various directions ( 262) and 

 varying in number and arrangement in 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 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. Which- 

 ever 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 proba- 

 bility, 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 



