INTR OD UCTION. 27 



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 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 muciu 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 dependent upon an af- 

 finity of the secreted substance for some material within the gland-cell. We 

 may indulge the hope that the details of the reaction will be discovered by 

 more complete chemical and microscopical 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 and 

 diffusion. The epithelial lining and its basement membrane form a septum 

 dividing the blood and lymph on the one side from the contents of the ali- 

 mentary canal on the other. Inasmuch as the two liquids in question are 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 concentration, into 

 the blood, where the concentration is less. Careful work of recent years has 

 shown that the laws of diffusion are not adequate to explain fully the ab- 

 sorption 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 in part by the vital activity of the epithelial cells. It will be 

 noticed that the vital property in this case is again a selective affinity for 

 certain constituents similar to that which has been referred to in the act of 

 secretion. The mere fact that the old mechanical theory has proved to be in- 

 sufficient is in itself no reason for abandoning all hope of a satisfactory ex- 



