INTRODUCTION. 3 



formed, and to what extent the animal cell is able to build it up for itself. 

 In such problems as these we meet with deep-seated differences in the 

 chemism of plant and animal cells. 



Physiology and physiological chemistry were at one time a single field 

 of investigation. The latter, owing to the remarkable progress of the 

 exact sciences chemistry and physics, has now developed to such an impor- 

 tant branch of natural science that it is altogether impossible to-day for 

 any one scientist to master thoroughly in all its details the whole field 

 of physiology. Little by little, there has developed a sharp distinction 

 between pure physiology and physiological chemistry. It is clear that 

 an artificial separation of two such closely related fields of investiga- 

 tion must work to disadvantage for the development of each, so that it is 

 fortunate to find it recognized more and more that the whole science of 

 physiology can only develop satisfactorily when there is an intimate 

 exchange of ideas between investigators in the two fields. The chemical 

 decompositions in the tissues do not take place beside the " physiological 

 processes," but are bound up with them. In no single case can we carry 

 out any distinction in such a sense; and with those organs in which the 

 relations between the functions and the metabolism are not known, this is 

 chiefly because of our limited knowledge concerning the latter. In this 

 connection we need only recall the whole nervous system. 



Finally we must mention the fact that physiological chemistry is con- 

 stantly approaching new fields of investigation which a few years ago 

 apparently had almost no connection with it. We have in mind here the 

 great field of infectious diseases and the changes in the metabolism of the 

 cells which they occasion. The apparently great gap existing between 

 the methods of protecting the organism against the substances produced 

 by micro-organisms and the products given off by the cells under normal 

 conditions is at last bridged over by means of certain analogies and numer- 

 ous transitions. Pathology also, in a broad sense, seeks to unite itself 

 more and more with the field of physiological chemistry. Many a path- 

 ological process has afforded us a desired physiological experiment, and 

 conversely it is possible to obtain by comparing physiological and path- 

 ological processes a clearer conception of the latter. It becomes more and 

 more evident that pathological changes in the tissues and cells should not 

 be judged entirely from a morphological standpoint. We can easily under- 

 stand that a disturbance in the metabolism of the cell, if continued long 

 enough, can make itself evident in the outer appearance. On the other 

 hand, it is perfectly conceivable that a definite functional disorder may 

 exist without the anatomist being able to localize it by the means at his 

 disposal. In no case does it follow necessarily that the limit of discern- 

 ible degeneration is a measure for any given functional disturbance. 

 Again, an abnormal morphological change may cause but a slight dis- 



