ABSORPTION OF THE DIGESTED FOOD. 351 



another and with the protoplasm of the epithelial cells, and in them, 

 through active movement of the protoplasm, wander the fat-granules, 

 which the cells take up and again give up within the villus. Thus, the 

 epithelial sheath, with the connective-tissue corpuscles of the villus, 

 forms the supporting apparatus ; the contents of the epithelial cells and 

 the numerous stroma-cells are the active propellers of the fat-granules 

 taken up. Through appropriate interstices in the tissues the cavities 

 containing the stroma-cells communicate with the axial lymph-vessel, 

 which is lined by endothelial cells. It is not improbable that leukocytes 

 frequently migrate from the capillary blood-vessels of the villus into 

 the tissue of the villus and, in part containing absorbed fat-granules, 

 pass over into the central lymph-vessel. According to Schafer, Zawary- 

 kin, Wiedersheim, Stohr, Preusse, Heidenhain and others, the ameboid 

 cells probably migrate from the parenchyma of the villi toward the 

 epithelial layer and perhaps even between the epithelial cells, and return 

 toward the axis of the villus, laden with the substances absorbed. 



A small artery enters every villus and, lying excentrically, passes 

 to the summit of the villus without division, to give off branches from 

 this point. In human beings this division begins at the middle. The 

 ramifications form a dense capillary network, which lies superficially 

 in the parenchyma of the villus, almost directly beneath the epithelial 

 layer, and from which, either at the apex of the villus or further downward, 

 a vein, running backward, is constituted. 



The villus is provided with unstriated muscular fibers, both deep- 

 seated, their bundles accompanying the central lymph- vessel longitu- 

 dinally, and also superficial, running rather transversely. 



The connective tissue of the small intestine has two layers, a deeper, composed 

 of thick, interwoven, mainly collagenous fibers (stratum fibrosum), and lying 

 above this a reticular layer intermixed with elastic fibers (stratum granulosum) , 

 entering into the villi also. 



Nerves enter the villi from Meissner's mucous-membrane plexus, are provided 

 with small, granular ganglion-cells in their course, and end in part in the muscles 

 of the villi and of the arteries, while in part they appear to communicate with 

 the contractile protoplasm of the epithelial cells. 



Nerve-filaments pass from Meissner's mucous-membrane plexus to the vessels 

 of the submucosa. Meissner's plexus communicates, by numerous fibers, with 

 a nerve-plexus that spreads throughout the entire thickness of the mucous mem- 

 brane, extends into the villi and supplies the muscularis mucosae, the vessels 

 of the mucosa and Lieberkuhn's glands. 



The epithelial cells of the large intestine possess no seam-like mar- 

 ginal thickening. 



The serous coat of the alimentary tract is provided with special 

 lymph- vessels, at first distinct from the chyle- vessels. 



ABSORPTION OF THE DIGESTED FOOD. 

 PHYSICAL FORCES: ENDOSMOSIS, DIFFUSION, FILTRATION. 



Endosmosis and diffusion take place between two liquids that are capable 

 of admixture, as, for example, hydrochloric acid and water, but never between 

 two fluids that are opposed to admixture, as, for instance, oil and water. If 

 two miscible dissimilar liquids are separated from each other by a membrane 

 provided with physical pores, such as may be present even in apparently homo- 

 geneous membranes, an interchange of the constituent parts takes place through 

 the pores of the membrane, until finally both fluids have the same composition. 

 This process is designated endosmosis or diosmosis. The endosmptic passage 

 of a substance through the membrane takes place if a solvent liquid having an 

 attraction for the substance is present on the other side of the membrane. 



