MECHANICAL MOVEMENTS IN CELLS. 



77 



motion, are not themselves, usually, the seat of any active change in form 

 although they slowly and gradually become longer or shorter, or perhaps 

 even divided. They are also capable of being entirely withdrawn into 

 the contractile body -mass. Such a form of motion, or rather the forms 

 resulting from such motion, are represented in Fig. 50. 



2. Ciliary Movement. — By ciliary movement is meant the pendulum- 

 like motion possessed by protoplasmic prolongations of fine hair-like 

 threads of numerous animal and vegetable cells. 



In many of the infusoria the entire external body surface, or a certain 

 limited portion of it, is supplied with minute hair-like appendages, which, 

 by their oscillation, serve as organs of propulsion. In vegetable spores 

 cilia are distributed in a similar manner, and likewise serve as propulsive 

 organs. 



Fig. 51.— Ciliated Epithelial Cells from the Nasal Mucous Mem- 

 brane of the Cow, Magnified 500 Diameters. (C. F. Muller.) 



In the animal kingdom ciliary movement is seen under numerous 

 forms on ciliated epithelial cells lining the nasal passages, antrum, tear- 

 duct and sacs, pharynx and Eustachian tube, middle ear, trachea, 

 bronchi, uterus and Fallopian tube, vas deflferens, epididymus, central 

 canal of the spinal cord and brain-ventricles, while cilia also serve as the 

 organ of movement in spermatozoa. 



The form of the cilium is, as a rule, that of a narrow, hair-like 

 thread. In all of the ciliated epithelial cells of the higher animals, as 

 well as in most spermatozoa, and in many of the lower animals and 

 plants, the length of such cilia may vary from 0.05 mm. to 0.005 mm. 

 (Fig. 51). They are structureless in appearance and colorless, and 

 possess a considerable amount of flexibility and elasticity. Under the 

 influence of various agents they may either swell up by imbibition or 



