LECTURE XXI. 

 THE FUNCTIONS OF THE DIGESTIVE ORGANS. 



I. 



WE have up to now considered the transformation of each separate 

 substance in the alimentary canal by itself, as well as its absorption, assimi- 

 lation, and subsequent destination until the final products of metabolism 

 were reached. Such a method of presentation has the advantage that it 

 gives us a clear idea concerning the behavior of any given foodstuff in 

 the animal organism, and makes it easier for us to trace the relations of 

 the separate organs to the remaining groups of foodstuffs and to their 

 functions. On the other hand, in order to avoid repetition, it was necessary 

 for us to touch only briefly upon certain very essential points, and, further- 

 more, there were certain important observations which we could not dis- 

 cuss at all. We shall now in the following lectures consider each individual 

 organ of the animal organism by itself, and in this way find opportunity 

 to mention what we have omitted, and, at the same time, to bind together 

 certain apparently isolated facts with other analogous ones to a single unit. 

 It is extremely difficult, and in fact impossible, to draw a sharp line between 

 pure physiology and physiological chemistry. The time has long since 

 passed when the latter branch of biological science could be considered as 

 concerned chiefly with the investigation of the composition of the separate 

 organs, the fluids of the body, and the excretions and secretions. It has 

 been recognized for some time that the tracing of the relations of the differ- 

 ent groups of foodstuffs to one another and their transformations in the 

 organism, has introduced new problems into the field of physiological 

 chemistry. With the solution of these problems, which are fundamentally 

 important for the understanding of the entire metabolism, the limits of the 

 working field of the physiological chemist are by no means reached, as we 

 shall see. Physiological chemistry takes part more and more in explaining 

 the functions of the various organs. Certain of the apparently- very-com- 

 plicated processes have been brought nearer to our comprehension by the 

 more recent investigations; and for some functions of which it was not 

 supposed that there was any relation to chemical processes, newer obser- 

 vations have opened up entirely new perspectives, especially from a chem- 

 ical point of view. Undoubtedly physiological chemistry must become 

 more closely united with pure physiology in order that the fruits obtained 

 in both fields may become fully ripe. 



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