very much diluted, and then probably pass straight into the lymph, and possibly also into the cells of the body. The optical method is not so exact as to permit us to establish very minute changes in rotation, and, even if it were possible to observe such rotations, it would be impossible to know for certain whether the fluctuations were not within the limits of errors of observation. Moreover, the decomposition undoubtedly proceeds quickly, so much so that we are really indebted to a lucky chance when we are able to follow up the decomposition of the injected matter in the plasma itself. These are the reasons why we have to prove the presence of the ferments by means of substrates, against which the respective ferments are directed. The substrate is the reactive for its corresponding ferment, and the decomposition of the former betrays the presence of the latter.
It may be remarked that the clear establishment of the presence of proteo- and peptolytic ferments in the blood plasma, after injection into the circulation of albuminous substances that are out of harmony with the body, has supplied a real explanation of the behaviour of parenterally injected proteins during metabolism. There is no longer any doubt that they are made use of, that is, that they are utilized in the metabolism of the cells of the body, so far as experience has shown decomposition to be
