INTRODUCTION. 27 



for a moment to examine briefly some of the problems which as yet have 

 escaped satisfactory solution by these methods. 



The phenomena of secretion and absorption form important parts of the 

 digestive processes in higher animals, and without doubt are exhibited in a 

 minor degree in the unicellular types. In the higher animals the secretions may 

 be collected and analyzed, and their composition may be compared with that 

 of the lymph or blood from which they are derived. It has been found that 

 secretions may contain entirely new substances not found at all in the blood, 

 as for example the mucin of saliva or the ferments and HC1 of gastric juice; 

 or, on the other hand, that they may contain substances which, although pres- 

 ent in the blood, are found in much greater percentage amounts in the secre- 

 tion — as, for instance, is the case with the urea eliminated in the urine. In the 

 latter case we have an instance of the peculiar, almost purposeful, elective 

 action of gland-cells of which many other examples might be given. With 

 regard to the new material present in the secretions, it finds a sufficient general 

 explanation in the theory that it arises from a metabolism of the protoplasmic 

 material of the gland-cell. It offers, therefore, a purely chemical problem 

 which may and probably will be worked out satisfactorily for each secretion. 

 The selective power of gland-cells for particular constituents of the blood is 

 a more difficult question. We find no exact parallel for this kind of action 

 in chemical literature, but there can be no reasonable doubt that the phe- 

 nomenon is essentially a chemical or physical reaction involving the activity 

 of some of the forms of energy with which the study of inanimate objects has 

 already made us partially familiar. We may indulge the hope that the details 

 of the reaction will be discovered by more complete chemical and micro- 

 scopical study of the structure of these cells. If in the meantime the act of 

 selection is spoken of as a vital phenomenon, it is not meant thereby that it is 

 referred to the action of an unknown vital force, but only that it is a kind of 

 action dependent upon the living structure of the cell-substance. 



The act of absorption of digested products from the alimentary canal was 

 for a time supposed to be explained completely by the laws of imbibition, 

 diffusion, and osmosis. The epithelial lining and its basement membrane form 

 a septum dividing the blood and lymph on the one side from the contents of 

 the alimentary canal on the other. Inasmuch as the two liquids in question 

 ;irc of unequal composition with regard to certain constituents, a diffusion 

 stream should be set up whereby the peptones, sugar, salts, etc. would pass 

 from the liquid in the alimentary canal, where they exist in greater concen- 

 tration, into the blood, where the concentration is less. Careful work of 

 recent years has shown that the laws of diffusion and osmosis are not adequate 

 to explain fully the absorption that actually occurs; a more detailed account 

 of the difficulties met with may be found in the section on Digestion and 

 Nutrition. It has become customary to speak of absorption as caused in part 

 by the physical laws of diffusion and osmosis, and in pari by the vital activity 

 of the epithelial cells. It will be noticed that the vital property in this case is 

 again an elective affinity for certain constituents similar to that which has been 



