

ON SOME OTHER FORMS OF CONTRACTILE TISSUE. 119 



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 momen- 

 tary 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 contraction 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 the ends 

 of tetanus are gained by a kind of contraction which, rare or at least not 

 prominent 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 deal- 

 ing with the heart itself. 



Ciliary Movement. 



89. Nearly all the movements of the body which are not due to physi- 

 cal causes, such as gravity, the diffusion of liquids, etc., are carried out by 

 muscles, either striated or plain ; but some small and important effects in the 

 way of movement are produced by the action of cilia, and by those changes 

 of protoplasm which are called amoeboid. 



90. Ciliary action, in the form in which it is most common, in mam- 

 mals and, indeed, vertebrates, consists in the cilium 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 movement is repeated 

 with very great rapidity so rapidly that the individual movements cannot 

 be seen ; it is only when, by reason of fatigue, the action becomes slow that 

 the movement itself can be seen ; what is seen otherwise is simply the effect 

 of the movement. The movements when slow have been counted at about 

 eight (double movements) in a second ; probably when vigorous they are 

 repeated from twelve to twenty times a second. 



The flexion takes place in one direction only, and all the cilia of each 

 cell and, indeed, of all the cells of the same epithelium move in the same 

 direction. Moreover, the same direction is maintained during the whole 

 life of the epithelium ; thus the cilia of the epithelium of the trachea and 

 bronchial passages move during the whole of life in such a way as to drive 

 the fluid lying upon them upward toward the mouth ; as far as we know, in 

 vertebrates, or at least in mammals, the direction is not and cannot by any 

 means be reversed. 



The flexion is very rapid, but the return to the erect position is much 

 slower ; hence the total effect of the blow, supposing the cilium and the cell 

 to be fixed, is to drive the thin layer of fluid in which the cilium is 

 working, and which always exists over the epithelium, and any particles 

 which may be floating in that fluid, in the same direction as that in which 

 the blow is given. If the cell be not attached* but floating free, the effect 

 of the blow may be to drive the cell itself backward ; and when perfectly 

 fresh ciliated epithelium is teased out and examined in an inert fluid, such 



