505 



by which the to-and-fro bending movement of the ordinary cilia of 

 Metazoa may be perfectly imitated. If we take a thin piece of india- 

 rubber tubing two or three inches long and alter the extensibility of 

 its wall along one side (which can be done either by rendering one 

 side more extensible by bringing it in contact with a very hot metal 

 surface or by having the tubing constructed with a strip of non-elastic 

 substance — cotton or linen — let into one side) and if we now attach 

 the tube to the nozzle of a syringe and having filled it and the syringe 

 with water, and closed the free end of the tube securely, increase the 

 fluid pressure within the tube, the latter bends over towards the less 

 extensible side exactly in the manner in which a cilium is bent over. 

 And on diminishing the pressure the straight position is at once again 

 taken up, and so by intermittently increasing and diminishing the fluid 

 pressure we obtain a working model which perfectly imitates ordinary 

 to-and-fro ciliary movement. Still more striking is the resemblance 

 if we have rendered the tube bulbous close to its attachment to the 

 syringe nozzle. For if this be done — the bulb also sharing the same 

 difference of extensibility on its opposite sides as the rest of the tube — 

 besides the general bend of the whole tube, there is a much sharper 

 bend at the bulbous part, as at a hinge, at once suggesting a mode of 

 movement which is very characteristic of many cilia. And in con- 

 nection with this, one cannot help recalling the fact that a bulbous 

 base of attachment is very commonly found as a structural feature of 

 the cilia of Metazoa. It is difficult to believe that the resemblance 

 between the action of such a model and the movement of cilia is 

 wholly coincidental. 



I would however by no means be considered to urge that merely 

 because we can make a physical model which perfectly imitates the 

 main features of ciliary motion, this fact by itself would furnish suf- 

 ficient evidence that the action and structure of cilia must necessarily 

 resemble the model. If there were no other evidence of such structure, 

 in the absence of any other conception of their mode of action which 

 is physically possible, in the absence also of any fact which would 

 render the tubular structure impossible of acceptance, we might fairly 

 accept the theory provisionally as furnishing a simple and adequate ex- 

 planation of ciliary action, which in so far as it involves a flowing of 

 the more fluid part of protoplasm coincident with local variations of 

 tension in the cell is so far strictly analogous to the only theory of 

 amoeboid activity which commends itself, I believe I am right in as- 

 serting, to the great majority of physiological thinkers. But the po- 

 sition is far stronger than this, for it is supported as we have seen 



