130 THE BODY AT WORK 



intestine, a very considerable proportion of the substance so 

 injected is absorbed. It is possible, indeed, to supply in this 

 way the whole of the nitrogenous food needed by the system, 

 none entering by the mouth. If milk is injected, a certain 

 amount of the fat also is retained. It can be shown that 

 such absorption takes place when no digestion of the food 

 occurs in the colon. The food is taken up by the epithelial 

 cells in the form in which it is injected. 



The organs specially devoted to absorption are the yilli, 

 which project into the contents of the small intestine. Each 

 is a conical process about 0-5 millimetre long. The villi 

 are longest in the upper half of the small intestine. Below 

 this level they decrease in number and size. A villus is 

 completely covered with epithelial cells of short, columnar 

 form. The free border of each cell is slightly hardened, 

 forming a disc or cap which appears striated in optical 

 section an indication, as some think, that it is traversed by 

 pores. Others hold that the appearance of striation is due to 

 minute cilia-like projections which beset the free border of 

 each cell. In worms and other invertebrates the cells carry 

 motile projections of not inconsiderable size, which no doubt 

 free their surfaces from the unassimilable matter which tends 

 to accumulate upon them. Possibly they help to fix particles 

 which are suitable for absorption. In mammals the presence 

 of cilia has not been demonstrated. The extreme minuteness 

 of the strise seems to point to their being merely indications 

 that the border is permeable to fluids, including droplets of fat. 



The so-called basement membrane upon which the epithelial 

 cells rest must not be regarded as a membrane in the physical 

 sense. Rather is it a basket-work which supports the cells, 

 without in any degree limiting their power of disgorging into 

 the lymph-spaces of the villi the substances which they have 

 absorbed. Within the villus, connective tissue forms a sponge- 

 work, the spaces of which are filled with lymph, in which a con- 

 siderable number of leucocytes roam, on the look-out, no doubt, 

 for any germs which may make their way between the epi- 

 thelial cells. In the centre of the villus is a lymphatic 

 radicle i.e., a fusiform cul-de-sac which is the dilated end 

 of a lymph- vessel. It, like all other lymph- vessels, is walled 

 by flattened endothelial scales. It communicates with the 



