192 FILTEATION AND CELL ACTION. 



terial which did not pre-exist in the blood. Thus, if it should be. shown 

 that, under normal circumstances, the elements of bile are not found in the 

 blood, the inference might be drawn that the hepatic cells display a com- 

 bining, or, as it were, a preparing power ; and so likewise in the case of 

 other secreting cells ; but the weight to be attached to such evidence is 

 greatly affected by the consideration that the action of each gland or se- 

 Difficuityofde- creting apparatus masks what is really going on in the sys- 



of C sec?etio"in S tem * ^ * s P 088 ^ 6 tnat we ma 7 l e scarcely able to discov- 

 the blood. er the traces of substances in the blood, and yet a tendency 

 may exist for their accumulation to a great extent. Thus there can be 

 no doubt that urea would abound through the disintegration of the mus- 

 cular structures, and the use of nitrogenized food, if it were not for the 

 action of the kidneys. It is the very perfection of that action which so 

 diminishes the amount in the circulation as to prevent us, except with 

 difficulty, from detecting the presence of the ingredient. 



Nor is this all, for it ought to be remembered that many of the prod- 

 ucts of secretion are substances undergoing retrograde metamorphoses, 

 and have therefore, as it were, in themselves, an interior principle of 

 change. It is conceivable that things which did not pre-exist in the 

 blood may yet occur in the secretions, coming tnere, not through the 

 agency of cell-life, but because of the downward course toward an inor- 

 ganic condition through which the secretion is spontaneously passing. 



Of the more prominent substances in the chief secretions, many indis- 

 putably pre-exist in the blood. Urea, cholesterine, casein, are examples. 

 Wherever this occurs, the removal is unquestionably due to mere filtra- 

 tion. Why should it be supposed that the cells of the kidneys have any 

 duty of combining material presented to them into urea, or those (if the 

 liver into cholesterine, or those of the mammary glands into casein ? As 

 our methods of examining the blood become more perfect, this formative 

 or grouping action, once so largely imputed to the secreting cells, be- 

 comes more and more restricted. 



The cases in which the influence of cells is indisputable are those which 

 Conditions of ^ er to us com ^ ma tions of progressive metamorphosis. Of 

 filtration and these, the most striking instance is the preparation of the sper- 

 of cell action. matic ^^ Perhaps we should not be very far from the truth 

 if we considered all those secretions in which the materials are in a state of 

 retrograde metamorphosis, or in a descending career, as arising by mere 

 filtration, and those which are ascending to a higher grade as due to cell 

 agency; between the two there being an intermediate class, the phase of 

 which is stationary, and in which cells may or may not be necessarily 

 involved, as, for instance, the transmutation of one fat into another, or 

 the preparation of sugar from albumenoid bodies. 



The apparatus for secretion is generally conveniently treated of under 



