MUSCULAR POWER. 115 



But a never failing indication of their presence is 

 obtained from the visible currents in the adjacent 

 fluid, which is driven along the surface when they 

 are in full activity. When the animal is at rest, 

 and attached to any object which prevents it from 

 moving in the fluid, these currents are apparent 

 wherever cilia are present, whether on the outer 

 surface, or in the internal cavities of the body. 

 But when the animal is detached, and at liberty to 

 move, the reaction of the fluid on which the cilia 

 strike, propels the body in the opposite direction, 

 and gives it a rapid progressive movement. The 

 motion of the individual cilia appears to be that 

 of simple flexion in one direction, and a return 

 to the original position. Some naturalists, how- 

 ever, have represented it as being circular, each 

 cilium describing a cone which has its apex at the 

 base.t 



On viewing a part which is fringed with cilia, 

 they frequently appear as if they were in progres- 

 sive motion along the edge of the part, an ap- 

 pearance which, when the cilia surround the margin 

 of a circular disk, gives rise to the illusion of its 

 having a rotatory motion ; for the disk then looks 

 exactly like a toothed wheel incessantly revolving 

 on its centre in one constant direction ; and the 

 deception is aided by the fluids being also carried 

 round in a continual vortex. This remarkable 

 phenomenon has excited much curiosity ; for the 

 slightest reflection is sufficient to convince us that 

 the continued revolution round an axis of any part 

 or appendage to the body is inconsistent with any 



f Purkinje, Valentin, and Ehrenberg. 



