MOTION 41 



time, the different cilia do not project vertically outward, but are 

 more or less bent. When contracting the cilium curves strongly toward 

 its vertical position, its convex border being at first strongly inclined 

 in this direction. Having reached its extreme position on the other 

 side of the vertical line, it returns to the position of rest by the process 

 of relaxation. The former movement is, of course, more rapid than 

 the latter and constitutes the effective stroke of the cilium. It is 

 accomplished by the contraction of the ciliary substance situated on 

 the side toward which the stroke is being directed, the opposite side 

 meanwhile being put on the stretch. The contraction having been 

 completed, the cilium is forced into its original position in consequence 

 of the elastic recoil of the stretched side. 



If a cell is beset with only one of these hair-like projections, an 

 interference with its motions is not likely to occur, but as there usu- 

 ally are a number of cilia situated upon a single cell, the question may 

 be asked how they can avoid beating against one another. Their 

 strokes are of course very rapid, so that the eye is scarcely able to 

 follow them. We thus obtain merely the impression of a general 

 motion which, however, it is possible to render more conspicuous by 

 adding some granular material to the medium in which they are 

 contained. The individual granules will then be forcibly thrown in 

 the direction of the effective stroke of the cilia. The character of 

 their beat may be studied more advantageously in preparations which 

 have been under observation for some time, because the movements 

 of dying cilia gradually become less rapid until eventually a number of 

 them may be found which beat only at intervals. Their movements 

 may also be considerably retarded by moistening them with a few drops 

 of ice-cold saline solution. Under ordinary conditions the cilia of the 

 frog's pharynx beat at the rate of 12 times in a second. Their con- 

 tractions, however, do not take place simultaneously but successively, 

 those in the front row of each field becoming active first, those in the 

 second next, and so on, until the last one has been involved. In 

 this way, it is brought about that the cilia of each field present all the 

 different stages of contraction and relaxation and give the impression 

 of regular waves passing over them. 



The regular sequence of these waves of contraction is not effected 

 with the aid of nervous structures, but is dependent upon a proto- 

 plasmic continuity between the different cells. Naturally, this 

 action arises in consequence of extraneous stimuli, but the impulses 

 themselves are generated in the cilium, or rather, in the cell to which 

 it is attached. That this is so, may be gathered from the fact that the 

 cilium ceases to beat, if broken off at its base, but continues to act if 

 left in contact with at least a small fragment of the cell in the vicinity 

 of its root. The contraction of the cilia takes place with rhythmic 

 regularity; moreover, since it occurs without the intervention of the 

 nervous system, it may be said to be automatic in its character. 



The function of the cilia is entirely mechanical, in that they impart 



