WITH THE MICROSCOPE. l6l 



it were, a vibration to and fro the alternate shortening and lengthen- 

 ing of a fibre. ' Contraction takes place in one definite direction only, 

 and never alters. Vital movement may occur in a mass of living 

 matter in any direction. See also p. 169. 



257. Ciliary Movement. Although this kind of movement is 

 not peculiar to animals, but is also found among plants, at least 

 during the early stages of existence of some of the lower forms, 

 it will be convenient to consider the methods of demonstrating cilia 

 in an active state of vibration in this place. 



Upon certain surfaces in the higher animals, and to a greater 

 extent in the lower classes, we find that the cells which generally 

 form the outer protective covering of more delicate structures, are 

 provided with very active vibratile processes, or cilia, which by their 

 movements create currents often of some considerable power. 

 These movements are sometimes required to promote the rapid 

 removal of foreign bodies which would injure delicate surfaces if 

 they came in absolute contact with them, or for promoting a constant 

 change in the water by which the animal is surrounded. Cilia 

 effect the latter object in the greater number of shell fish, which 

 are stationary throughout life, and are not provided with an apparatus 

 for promoting a continual change of the fluid Avhich bathes the 

 surface of their respiratory organs. 



Ciliary Motion endures for a longer or shorter period after death, 

 and is entirely independent of the nervous system. In the active 

 bird it ceases very soon, but in the more slowly nourished, cold- 

 blooded animals it often lasts for many days after death. 



Different forms of ciliary action may be observed among the 

 different species of infusoria. It is, however, doubtful if many of 

 the very fine spine-like bodies, the movements of which seem to be 

 under voluntary control, should be regarded as cilia. The simple 

 organisms of this class seem to possess the power of stopping the 

 vibrations, although there can be no doubt that in vertebrate animals 

 ciliary action is quite independent of volition. There is certainly 

 no connexion between the cells of ciliated epithelium and the 

 nerves. 



. Cells with ciliated epithelium in active vibration can always be 

 obtained by scraping the back of the frog's tongue. Mucus is re- 

 moved in which numerous cells are found. The thin glass cover 

 must be prevented from pressing too firmly by inserting a small piece 

 of thin paper beneath it. The student may also obtain very beautiful 

 ciliated epithelium in active vibration from the branchiae (gills) of 

 the oyster or mussel. Some of the cilia from the latter situation 

 are of very considerable length, and occasionally the vibration of a 



M 



