338 Prof. J. S. Macdonald. [Apr. 17, 
can be considered as an explanation of a hitherto unexplained condition. 
Its existence diminishes the value which might otherwise be placed upon 
the measurements of the electrical conductivity of colloid solutions. They 
cannot be used as objections to the assumed method of arrangement of 
inorganic salts in the colloid solution of the nerve-fibre, in which there is 
good reason to suppose that “pseudo-polarisation” is an easily elicited 
phenomenon. The hypothesis may therefore meet with serious considera- 
tion, and with it the conception that such colloid solutions as exist in the 
nerve-fibre form ample stores of energy in the manner suggested. 
There are of necessity limits to the part which it is suggested these stores 
of potential energy play in the redistributions of energy observed in living 
tissues. In the first place, it is not suggested that the extreme amount 
of energy accumulated by them is in every case expended. Hitherto, in 
the case of nerve injury, we have been considering the extreme case, the 
generally fatal accident, in which the primary conditions essential to storage 
have been suddenly and completely annihilated. It is reasonable to suppose 
that such extreme instances of coagulation are not of normal occurrence, and 
that, within the living organism, departures from the normal state are only 
permitted within readily reversible limits. 
Let us consider the case of a simple cellular unit, one of the many 
other similar units placed side by side in the formation of a tissue. Let 
us take the case of a single ciliated epithelial cell. Within the cell itself 
are certain surfaces limiting the movements of salts, but permitting a 
movement of water. On one side of one of these surfaces inorganic salts are 
suddenly liberated, there is a translation of water across the surface, the cilia 
are rendered turgid and are moved. Further, the movement of molecules 
of inorganic salts, although limited, is not entirely prevented; there is a 
tendency for diffusion to take place from this cell into its neighbour, the two 
cells are thereby oppositely charged; the electrical charge bestowed upon 
the neighbouring cell excites this in its turn; the charge left in the original 
cell brings its colloid solution back into, or nearly into, their normal state. 
The cell, therefore, is excited, performs its function, excites its neighbour, and 
itself returns to rest. In this we have an essential unit of a picture of ciliary 
motion traversing a sheet of epithelium in a definite direction. We have 
not, however, the whole of the essential unit. In the performance of this 
cycle of chanye external work has been performed, the store of potential 
energy has therefore been necessarily reduced. To succeed absolutely in 
returning to its original state, the cell must necessarily resort to a transac- 
tion with some external source of energy. True, the amount required may 
not be as great as at first sight appears, since there has been a give and take 
