CH. XXXIV.] PHYSIOLOGICAL FACTOR IN ABSORPTION 523 



with bile allow fatty materials to pass through them more readily 

 than would otherwise be the case. In cases of disease in which bile 

 is absent from the intestines, a large proportion of the fat in the food 

 passes into the faeces. 



Since the days of Lieberkiihn it has been the desire of physiologists to prove 

 that the absorption of solutions from the intestines can be explained upon some 

 simple physical basis. Thus the processes of filtration, osmosis, and imbibition, 

 either alone or in combination, have been in turn called upon as affording the 

 requisite explanation. Such theories have alternated with others in which the 

 physical cause has been either wholly or in part rejected as inadequate, and the 

 deficiencies of the physical cause supplemented by the physiological, vital, or 

 selective action of the epithelial lining of the alimentary tract 



The difficulty of the problem does not, however, entirely depend on the impos- 

 sibility of defining the word vital, but also on the complicated nature of the physical 

 processes to which we have alluded. Since the days when Fischer and Dutrochet 

 inaugurated our elementary knowledge of osmotic phenomena, a great amount of 

 research has been expended in making that knowledge more accurate, but even 

 at the present day it is doubtful whether all the aspects of the question are fully 

 understood (see also p. 321). The subject has, in recent years, been taken up by 

 Waymouth Reid, who has made a life-study of such phenomena, and whose work 

 must be regarded as authoritative. 



The animals he experimented on were dogs, and the material selected for 

 absorption was the serum or plasma of the blood from the same animals. The sub- 

 stances to be absorbed were thus of the same kind as those in the blood and lymph 

 on the other side of the absorptive epithelium. The serum or plasma was analysed, 

 introduced into an isolated loop of the gut, and at the end of a given time the con- 

 tents of the loop were again analysed. The pressure in the loop and in the mesen- 

 teric veins was estimated manometrically during the progress of the experiment ; 

 allowance was made for the secretion of intestinal juice, and other precautions 

 taken to make each experiment as complete as possible. 



It was found that the absorption by an animal of its own serum or plasma 

 takes place under conditions in which filtration or osmosis into blood capillaries or 

 lacteals and also adsorption (or imbibition) are excluded. The active force must 

 therefore by a process of exclusion reside in the physiological activity of the lining 

 epithelium. The same conclusion was reached by another method, namely, that 

 when the epithelium is removed, injured or poisoned, the absorption either ceases or 

 is markedly lessened, and this in spite of the fact that removal of the epithelium 

 must increase the facilities for osmosis and filtration. 



The activity of the cells is characterised by a slower uptake of the organic solids 

 of the serum than of water, and a quicker uptake of the salts than of the water ; 

 but the absolute numerical relations vary in different regions of the intestine. The 

 state of nutrition of the cells is the main factor in their activity ; specific absorptive 

 nerve-fibres were sought for, but not found. The absorption of water from the gut 

 depends partly on the physical relation of the osmotic pressure of the solution in the 

 intestine to that of the blood plasma ; but even the absorption of water is influenced 

 by the physiological regulation of this difference by the directing or, as it may be 

 termed, orienting mechanism of the cells. Such orienting action was first noted in 

 connection with salts by Otto Cohnheim ; he showed that, in an intestinal loop with 

 injured cells, sodium chloride enters its lumen from the blood though the same salt 

 is being actively absorbed from a normal loop in the same animal at the same time. 

 In all probability the cell activity which causes the organic constituents of serum to 

 pass into the blood is of the same nature as that involved in the orienting action of 

 the cells upon salts in solution. 



Reid's conclusions with regard to the absorption of peptone and sugar are 

 as follows: The chief factor in the absorption of peptone is an assimilation (or 

 absorption) by the cells, while in the absorption of glucose diffusion variable by the 

 permeability of the cells (and so probably related to their physiological condition) is 



