THE ABSOKPTION OF FOOD. [CH. xxxiv. 



though advantageous for the" formation of these substances, is 

 not essential. 



We thus see how with increase of knowledge due to improved 

 methods of research, a complete change has come in the ideas 

 physiologists hold regarding this subject. It is not so many 

 years ago, that the physical change emulsification which fats 

 undergo in the intestine was considered to be more important 

 than the chemical changes fat-splitting and sapon in cation. In 

 fact, the small amount of chemical change which was supposed 

 to occur was regarded as quite subordinate, and of value merely 

 in assisting the process of emulsification. We now know that the 

 exact converse is the truth ; the chemical change is the important 

 process, and emulsification the subordinate one. 



Bile aids the digestion of fat, in virtue of its being a solvent of 

 fatty acids, and it probably assists fat absorption by reducing the 

 surface tension of the intestinal contents; membranes moistened 

 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 turns 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 supple- 

 mented by the physiological action of the epithelial lining of the alimentary 

 tract. The terms ;i vital selection " or " physiological action of living cells " 

 are admittedly unsatisfactory. Tliey afford no real explanation of the 

 intimate nature of the process ; they are provisional names for what cannot 

 yet be brought into line with precise physical and chemical data, but that 

 such processes do exist is undeniable, and so we must be content for the 

 present with the way in which they are labelled. 



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

 impossibility 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, and at the present day it requires almost a 

 specialist in this branch of physics to thoroughly understand it. Even so 

 eminent a man as the late Professor Heidenhaiii did not fully comprehend 

 the nature of osmotic processes, and his epoch-marking work on absorption 

 consequently suffers. The subject has more recently 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 substances 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 



