318 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY. 



i- concerned, the process of absorption must be of a mechanical nature. The 

 details of the process have been worked ouf microscopically and have given 

 rise to numerous researches. It is unnecessary to speak of the various 

 theories that have been held, as it has been shown by nearly all the recent 

 work that the immediate agent in the absorption of fats is again the epi- 

 thelial cells of the villi of the small intestine. The fat-droplets may be 

 seen within these cells, and can be studied microscopically alter digestion 

 in the act of passing, or rather of being passed, through the cell-substance. 

 Reference to the histology of the villi will show that each villus possesses 

 a comparatively large lymphatic capillary lying in its middle and ending 

 blindly, apparently, near the apex of the villus. Between this central lym- 

 phatic — or lacteal, as it is called here — and the epithelium lies the stroma, or 

 main substance of the villus, which, in addition to its blood-capillaries and 

 plain muscle-fibres, consists mainly of lymphoid or adenoid tissue containing 

 numerous leucocytes. The fat-droplets have to pass from the epithelium to 

 the central lymphatic, for it is one of the most certain facts in absorption, and 

 one which has been long known, that the fat absorbed gets eventually into 

 the iacteals in an emulsified condition and thence is conveyed through the 

 svstem of lymphatic vessels to the thoracic duct and finally to the blood. 

 The name " lacteal," in fact, is given to the lymphatic capillaries of the villus 

 on account of the milky appearance of their contents, after meals, caused by 

 the emulsified fat. It should be added, however, that it has not been possible 

 to demonstrate experimentally that all the absorbed fat passes into the thoracic 

 duct. Attempts have been made to collect all the fat passing through the 

 thoracic duct after a meal containing a known quantity of fat, but even after 

 making allowance for the unabsorbed fat in the feces there is a considerable 

 percentage of the fat absorbed that cannot be recovered from the lymph 

 of the thoracic duct. While this result does not invalidate the conclusion 

 stated above that the fat passes chiefly, perhaps entirely, into the Iacteals, 

 it does indicate that there arc some factors concerned in the process of fat- 

 absorption that are at present unknown to us. The passage of the fat- 

 droplets to the central lacteal is not difficult to understand. The adenoid 

 tissue of the stroma is penetrated by minute unformed lymph-channels that 

 are doubtless connected with the central lacteal. In each villus lymph is 

 continually formed from the circulating blood, so that there must be a slow 

 stream of lymph through the stroma to the lacteal. When the fat-droplets 

 have passed through the epithelial cells (and basement membrane) they drop 

 into the interstices of the adenoid tissue and are carried in this stream into 

 the lacteal. The Iacteals were formerly designated as the "absorbents," under 

 the false impression that they attended to all the absorption going on in the 

 intestines, including that of peptones, sugars, and fats. It is now known that 

 their action under ordinary conditions is limited to the absorption of fats. 



Absorption of "Water and Salts. — From what has been said (p. 312) it is 

 evident that absorption of water takes place very slightly, if at all, in the 

 -o.inael). Whenever soluble substances, such as peptones, sugars, or salts, are 



