162 CILIARY MOVEMENT. [BOOK i. 



intestine or of other organs supplied with plain muscular fibres, 

 even in the case of the frog, for any long period after removal 

 from the body. 



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 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 dealing with the heart itself. 



Ciliary Movement. 



93. 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 protoplasm which are called 

 amoeboid. 



Cilia are generally appendages of epithelial cells. An epithelium 

 consists of a number of cells, arranged in a layer, one, two or more 

 cells deep, the cell bodies of the constituent cells being in contact 

 with each other or united merely by a minimal amount of cement 

 substance, not separated by an appreciable quantity of intercellular 

 material. As a rule no connective tissue or blood vessel passes 

 between the cells, but the layer of cells rests on a basis of vascular 

 connective tissue, from which it is usually separated by a more or 

 less definite basement membrane, and from the blood vessels of 

 which its cells draw their nourishment. The cells vary in form, 

 and the cell body round the nucleus may be protoplasmic in 

 appearance or may be differentiated in various ways. An epithe- 

 lium bearing cilia is called a ciliated epithelium. Various passages 

 of the body, such as, in the mammal, parts of the nasal chambers 



