EPITHELIUM; CILIARY MOVEMENT. 



145 



another is produced, as when a corn-field is agitated by frequent gusts. 

 When the Ciliary motion is taking place in full activir^, however, nothing 

 whatever can be distinguished, but the whirl of particles in the surrounding 

 fluid ; and it is only when the rate of movement slackens, that the shape 

 and size of the Cilia, and the manner in which their stroke is made, can be 

 clearly seen. The motion of the Cilia is not only quite independent (in all 

 the higher animals at least) of the will of the animal, but is also independent 

 even of the life of the rest of the body ; being seen after the death of the 

 animal ; and proceeding with perfect regularity in parts separated from the 

 body. The isolated epithelium cells have been seen to swim about actively 

 in water, by the agency of their cilia, for some hours after they have been de- 

 tached from the mucous surface of the nose ; and the Ciliary movement has 

 been seen fifteen days after death in the body of a Tortoise, in which putre- 

 faction was far advanced. In the gills of the River-Mussel, which are among 

 the best objects for the study of it, the movement endures with similar per- 

 tinacity. 



172. The purpose of this Ciliary movement is obviously to propel fluids 

 over the surface on which it takes place ; and it is consequently limited in 

 the higher animals to the internal surfaces of the body, and always takes 

 place in the direction of the outlets, towards which it aids in propelling the 

 various products of secretion. The case is different, however, among animals 

 of the lower classes, especially those inhabiting the water. Thus the external 

 surface of the gills of Fishes, Tadpoles, &c., is furnished with cilia; the con- 

 tinual movement of which renews the water in contact with them, and thus 

 promotes the aeration of the blood. In the lower Mollusca, and in many 

 Zoophytes, which pass their lives rooted to one spot, the motion of the Cilia 

 serves not merely to produce currents for respiration, but likewise to draw 

 into the mouth the minute particles that serve as food. [Fig. 107, 2, 5.] 

 And in the free-moving Animalcules, 



of various kinds, the Cilia are the [Fig. 40. 



sole instruments which they possess, 

 not merely for producing those cur- 

 rents in the water, which may bring 

 them the requisite supply of air and 

 food, but also for propelling their 

 own bodies through the liquid ele- 

 ment. This is the case, too, with 

 many larger animals of the class 

 Acalepha (Jelly fish), which move 

 through the water, sometimes with 

 great activity, by the combined ac- 

 tion of the vast numbers of cilia, that 

 clothe the margins of their external 

 surfaces. In these latter cases, it E ^ m P'of cnia; i, portionof a barof the g.iiof 



c i /~ci- sea-mussel, Mytilus eaulis, showing cilia at rest 



biliary move- and in motion . 2 , ciliated epithelium particles from the 

 frog's mouth; 3. ciliated epithelium particles from in- 

 ner surface of human membrana tympani ; 4, ditto, 



ment were more under the control 

 of the will of the animal, than where 



ditto, from the human bronchial mucous membrane 

 5, Leucophrys patula, a polygastric infusory animal- 



it is concerned only in the organic 



functions. In what Way the will Cll | e; , B how its surface covered With cilia, and the 

 Can influence it, however, it does mouth surrounded by them.] 



not seem easy to say ; since the 



ciliated epithelium-cells appear to be perfectly disconnected from the surface 

 on which they lie, and cannot, therefore, receive any direct influence from 

 their nerves. Of the cause of the movement of the Cilia themselves, no ac- 

 count can be given ; they are usually far too small to contain even the minutest 

 13 



