To return to the sanitary engineer whose opinion we asked regarding the mode of working of the drainage system of the kidney. Probably he would deny that the problems came within his province. “They are not physical, but vital,” he would say. “I know nothing about the vital action of the cells which line the tubule.” Objection may be taken to the form of expression, albeit he was fully justified in declining to discuss the question any further. He does not know enough about the internal structure of a cell to be able to predict the phenomena of osmosis which will occur within it. No one can say what capacity living cells may have of taking substances from the blood, returning some of them, and excreting others. This unknown capacity leads to results which, when they do not appear to be in accordance with the laws of physics, are commonly termed “vital.” The term is a stumbling-block which has tripped up generations of physiologists. The expressions “vital action” and “physical phenomena” have been used as if they were antithetical, whereas all vital actions are physical phenomena. “Vital” in this sense connotes “as yet unknown.” Yet, in truth, there is abundant excuse for the use of a term which covers ignorance, so long as its connotation is not extended until it assumes a positive, antiphysical sense. “Physical” and “vital” are expressions which point a contrast constantly present to a physiologist’s mind. He knows perfectly well that the passage of water and salts through a membrane, and their passage into and out of a living cell, are equally phenomena of osmosis. But the former process he can test and measure in his laboratory; the latter he can but observe in much obscurity in the living body. He cannot make a model of a living cell. In the case of the salivary gland, as we have already seen, living cells take water from lymph, and discharge it as saliva in apparent opposition to osmotic force. They reverse the direction of the flow which would occur were lymph and saliva separated by a membrane. But a cell is not a membrane. It is an extremely complicated structure with an elaborate architecture of its own. As well might we compare the distribution of water by a County Council water-cart and its passage through a brewery. According to all the laws of hydrostatics, the water which flows into a brewery should leave it through its drains. Its exit in barrels on drays is antiphysical. When the physiologist can explore the living cell, he will discover that the imbibition and extrusion of water, the selection, retention, and discharge of salts, are phenomena as strictly physical as their passage through a dialyser in his laboratory. In the meantime he can but contemplate the cell with a certain degree of awe. His best devised model of a urinary tubule may lead him into error, for the simple reason that he cannot line it with living cells. A living cell has a power which upsets all calculations, falsifies all experimental findings.