The sugar-making faculty of the liver is another “catalytic” mystery, as great as the rest of them, and no greater.  Liver-tissue brings sugar out of the blood, or out of its own substance;—­why?

     Quia est in eo
     Virtus saccharitiva.

Just what becomes of the sugar beyond the fact of its disappearance before it can get into the general circulation and sweeten our tempers, it is hard to say.

The pancreatic fluid makes an emulsion of the fat contained in our food, but just how the fatty particles get into the villi we must leave Brucke and Kolliker to settle if they can.

No one has shown satisfactorily the process by which the blood-corpuscles are formed out of the lymph-corpuscles, nor what becomes of them.  These two questions are like those famous household puzzles,—­Where do the flies come from? and, Where do the pins go to?

There is a series of organs in the body which has long puzzled physiologists,—­organs of glandular aspect, but having no ducts,—­the spleen, the thyroid and thymus bodies, and the suprarenal capsules.  We call them vascular glands, and we believe that they elaborate colored and uncolored blood-cells; but just what changes they effect, and just how they effect them, it has proved a very difficult matter to determine.  So of the noted glandules which form Peyer’s patches, their precise office, though seemingly like those of the lymphatic glands, cannot be positively assigned, so far as I know, at the present time.  It is of obvious interest to learn it with reference to the pathology of typhoid fever.  It will be remarked that the coincidence of their changes in this disease with enlargement of the spleen suggests the idea of a similarity of function in these two organs.

The theories of the production of animal heat, from the times of Black, Lavoisier, and Crawford to those of Liebig, are familiar to all who have paid any attention to physiological studies.  The simplicity of Liebig’s views, and the popular form in which they have been presented, have given them wide currency, and incorporated them in the common belief and language of our text-books.  Direct oxidation or combustion of the carbon and hydrogen contained in the food, or in the tissues themselves; the division of alimentary substances into respiratory, or non-azotized, and azotized,—­these doctrines are familiar even to the classes in our high-schools.  But this simple statement is boldly questioned.  Nothing proves that oxygen combines (in the system) with hydrogen and carbon in particular, rather than with sulphur and azote.  Such is the well-grounded statement of Robin and Verdeil.  “It is very probable that animal heat is entirely produced by the chemical actions which take place in the organism, but the phenomenon is too complex to admit of our calculating it according to the quality of oxygen consumed.”  These last are the words of Regnault, as cited by Mr. Lewes, whose intelligent discussion of this and many of the most interesting physiological problems I strongly recommend to your attention.