86 STRUCTURE OF THE VILLI. 



vessels. Their epithelial covering of cylindroid cells is shown in the 

 sectional diagram, Fig. 27, a a ; at b b is the origin of the lacteal aris- 

 ing obscurely. ' Fig. 27. 



So amply are the villi supplied 

 with blood-vessels, that if, after in- 

 Various opin- jection with coloring 



materia1 ' their c J lin - 



cells. dric epithelium be re- 



moved, they seem to be tinged all 

 over. Each cell of the epithelium 

 appears to be filled with granular 

 matter, and to have a well-marked 



nucleus. Some anatomists assert Conical villi in section, with cylindroid epithelium. 



that that end of these cells nearest the cavity of the intestine is in reality 

 open, and in this manner they account for the ready passage of oil glob- 

 ules into them, and also for the appearance of solid foreign bodies, as Os- 

 terlein observed. 



Though we have described the lacteal as a vessel projecting into the 

 Origin of the interior of the intestine, it is by some viewed rather as 'a mere 

 lacteal. excavation in the villus. The villi impart to the mucous mem- 



brane an aspect sometimes likened to the pile of velvet. On an average, 

 their number upon a square inch is about 10,000. The entire number 

 of these organisms must, therefore, amount to many millions. At one 

 time it was supposed that the lacteals open directly into the intestine an 

 opinion which is now universally abandoned. The action of each villus is 

 doubtless more complicated than is generally represented, for the organic 

 fibre cells it contains give to it the power of executing rhythmic motions. 



When the operation of the lacteal vessels as absorbents was first de- 

 The lacteals tected, it was believed that all nutriment is introduced by 



Sfc^anfef t ^ ieir means - But tnere are man j animals wholly destitute 

 absorption. of this system of tubes, for instance, the invertebrates. Even 

 in many fishes the villi are absent. In such cases absorption must nec- 

 essarily be conducted by the veins. Moreover, though there are no lac- 

 teals on the walls of the stomach, nor, indeed, on that part of the intes- 

 tinal tube which is higher than the place of introduction of the biliary 

 and pancreatic ducts, there are many substances freely absorbed from the 

 gastric cavity when its pyloric orifice is tied. It has already been men- 

 tioned that the stomach absorbs water with remarkable rapidity. The 

 doctrine that the lacteals are the exclusive organs of absorption must, 

 therefore, be abandoned, for it is plain that the venous system participates 

 in this duty. 



The function of absorption has therefore to be examined from two 

 points of view. As there are two digestions, one producing a perfect so- 



