516 SPECIAL HISTOLOGY. 



absorption commences, but up to tbis time nothing bas entered tbe lac- 

 teals. This, however, soon takes place, and the first indication we ob- 

 serve, is the breaking up of the large drops of fat in the cells into many 

 tolerably minute fatty molecules. When this has occurred, these drops 

 penetrate by degrees, from all sides, into the parenchyma of the villus 

 itself, fill it more and more and, at last, enter the central lacteal, whose 

 whole length they eventually occupy. In the meanwhile, fresh fat has 

 been continually passing in from the intestinal canal, not in the form of 

 large drops, however, but henceforward, in small molecules or drops of 

 the same kind as those which were at first developed secondarily in the 

 cells. On the other hand, at a subsequent period, we not uncommonly 

 meet, in the interior of the villi, with large round drops, which appear 

 especially inclined to form considerable accumulations at their apex. 

 In man, I have not yet had the opportunity of tracing the process of 

 the absorption of fat, step by step, but we may here so frequently ob- 

 serve, on the one hand, cylindrical epithelial cells filled with fatty mole- 

 cules, and on the other, collections of larger and smaller drops of fat 

 in the parenchyma of the villi, especially at their points and in their 

 axes, that I do not at all hesitate to suppose the process to be the same 

 as in animals, without, however, wishing to imply that all the steps are 

 identical. These observations demonstrate that fatty matters are ab- 

 sorbed as such and are not saponified ; on the other hand, it cannot at 

 present be certainly stated how it is possible that they penetrate the 

 membrane of the epithelial cells, the parenchyma of the villi, and the 

 walls of the lacteals. I should be most inclined to compare the whole 

 process to the imbibition of an emulsive fluid, such as milk, by a porous 

 body ; and I believe that the fatty molecules of the chyme are absorbed 

 simply in consequence of their being carried along with its fluid part.* 



* [Professor Brucke, in a series of important papers on the " Lacteals and on the Absorp- 

 tion of Chyle" (Ueber die Chylusgefftsse und die Resorption des Chylus), published in the 

 Transactions of the Vienna Academy for 1854, has thrown much light on the difficult 

 question of the anatomy and physiology of the-parts concerned in digestive absorption. 



As the result of his observations on Man and on the Mammalia, Brucke states, that the 

 cylindrical epithelial cells covering the villi, are open at one extremity, and are not, as is 

 generally supposed, closed by a membrane, but merely by a thin layer of a mucilaginous 

 substance. This, he asserts, can be proved by observations on animals recently killed; for 

 if a portion of their intestinal mucous membrane be brought under the field of the micro- 

 scope, the formation of transparent vesicles, corresponding to each epithelial cell may be 

 easily seen. These vesicles form along the margin of the villus, and are the cell-contents 

 which have escaped from the open extremity of the cylindrical cell, enclosed by the muci- 

 laginous substance. They have hitherto been regarded as the cell- wall rendered prominent 

 by an expansion of the cell-contents (vid. supra, Fig 211, C a) ; but that this view is in- 

 correct, is evident from the circumstance, that they lie perfectly isolated by the sidje of the 

 cells, without any change in the membrane of the latter being observable. 



As another evidence of these epithelial cells not being closed cavities, Professor Brucke 

 adduces the fact, that even by the aid of the highest magnifying powers, no cell membrane 

 with pores can be distinguished. Yet the oil globules, which are always of a distinctly re- 

 cognizable size, are known to enter the cells, and it is difficult to understand how they can 

 traverse a homogeneous membrane. 



