134 CILIARY MOVEMENT. [BOOK i. 



ments of parts provided with plain muscular fibres when we come 

 to consider the parts themselves. 



Like the skeletal muscles, whose nervous elements have been 

 rendered functionally incapable ( 73), plain muscles are much 

 more sensitive to the making and breaking of a constant current 

 than to induction shocks ; a current, when very brief, like that of 

 an induction-shock, produces little or no effect. 



The contraction of plain muscular fibres is as we said very slow 

 in its development and very long in its duration, even when started 

 by a momentary stimulus, such as a single induction-shock. The 

 contraction after a stimulation often lasts so long as to raise the 

 question, whether what has been produced is not a single contrac- 

 tion but a tetanus. Tetanus, however, that is the fusion of a series 

 of contractions, seems to be of rare occurrence, though probably it 

 may be induced, in plain muscular tissue ; but some of the ends 

 of tetanus are gained by a kind of contraction which, not prom- 

 inent in skeletal muscle, becomes of great importance in plain 

 muscular tissue, by a kind of contraction called a tonic contraction. 

 The subject is one not without difficulties, but it would appear that 

 a plain muscular fibre may remain for a very considerable time in 

 a state of contraction, the amount of shortening thus maintained 

 being either small or great: it is then said to be in a state of 

 tonic contraction. This is especially seen in the case of the plain 

 muscular tissue of the arteries, and we shall have to return to this 

 matter in dealing with the circulation. 



The muscular tissue which enters into the' construction of the 

 heart is of a peculiar nature, being on the one hand striated, and 

 on the other in some respects similar to plain muscular tissue, but 

 this we shall consider in dealing with the heart itself. 



Ciliary Movement. 



85. Nearly all the movements of the body which are not due 

 to physical causes, such as gravity, the diffusion of liquids &c., are 

 carried out by muscles, either striated or plain ; but some small 

 and yet important effects in the way of movement are produced 

 by the action of cilia, and by those changes of form which are 

 called amoeboid. Cilia are generally appendages of epithelial 

 cells. 



86. Ciliary action, in the form in which it is most common 

 in mammals and indeed vertebrates, consists in the cilium (i. e. the 

 tapering filament spoken of above) being at one moment straight 

 or vertical, at the next moment being bent down suddenly into a 

 hook or sickle form, and then more slowly returning to the straight 

 erect position. When the cilia are vigorous, this double move- 

 ment is repeated with very great rapidity, so rapidly that the 

 individual movements cannot be seen ; it is only when, by reason 



