CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 523 



If, however, we are thus driven to the conclusion that the 

 passage from the blood into the intestine is a manifestation of 

 secretory activity in which epithelium cells play a part, gradually 

 becoming little by little more intelligible to us, why should we 

 not admit that the passage from the intestine to the blood, which 

 as we have seen does not accord in its phenomena with known 

 processes of ordinary diffusion, is also brought about by the 

 activity of cells, is in fact a kind of inverted secretion, and 

 hence like ordinary secretion presents problems which cannot be 

 solved by any off-hand references to known physical processes? 

 Indeed this is the conclusion towards which observation and 

 experiment seem to be steadily leading us. Were the alveolus of 

 a salivary gland habitually filled with a fluid of mixed and varied 

 nature like the contents of the alimentary canal, we should 

 probably in our study of the gland find ourselves compelled to 

 speak of a double current as existing in the gland, of a current 

 from the cells to the lumen of the alveolus, and of a current from 

 the lumen to the cells. And all along the intestine both the 

 columnar and cubical cells, which everywhere bear the marks of 

 being " active " cells, may perhaps be regarded as engaged in a like 

 double function. Over the villi the receptive function, in the 

 glands of Lieberkuhn the ejective function is predominant ; but as 

 we have suggested, 265, in the glands .reception probably is not 

 wholly absent, and we may imagine that in the villi some 

 amount of ejection (quite apart from the action of the goblet 

 cells) may take place. 



If this view be accepted, if we admit that the entrance of 

 digested food does not take place by ordinary diffusion, the question 

 may be asked why are the digestive changes directed towards 

 increased diffusibility, why are proteids converted into diffusible 

 peptones, and why is starch converted into sugar? Because though 

 the cell is not an apparatus for diffusion, diffusion is an instrument 

 of which the cell makes use. When we say that peptone does not 

 enter the blood by ordinary diffusion we do not mean that diffusion 

 has nothing to do with the matter. The activity of a living cell is 

 an activity, built up upon and making use of various chemical and 

 physical processes; in it the processes of ordinary diffusion play 

 their part as do the processes of ordinary chemical decomposition ; 

 but the cell uses and modifies them for its own ends. If as we 

 have every reason to believe the cell of a villus passes the sugar 

 unchanged from the intestine into the blood capillary, it makes use 

 of diffusion to effect that passage ; and if it does change the proteid 

 into something else before it passes it on, it receives it into itself in 

 the first instance by help of diffusion. When we say that substances 

 do not enter the blood by ordinary diffusion we mean that the dif- 

 fusion which takes place in a living cell is something so different in 

 the results from ordinary diffusion through a dead membrane that 

 it is undesirable to speak of it by the same name. In ordinary 



