CILIARY MOVEMENTS. 461 



serve as a means of progression and are more rapid. Such movements are 

 called amoeboid. It does not seem possible to explain the nature and cause 

 of the movements of homogeneous contractile substance ; and it must be ex- 

 cessively difficult, if not impossible, to observe directly the effects of differ- 

 ent stimuli, in the manner in which the movements of muscles are studied. 

 They seem to be analogous to the ciliary movements, the cause of which is 

 equally obscure. 



Ciliary Movements. The epithelium covering certain of the mucous 

 membranes is provided with little, hair-like processes upon the borders of 

 the cells, called cilia. These are in constant motion, from the beginning to 

 the end of life, and they produce currents upon the surfaces of the mem- 

 branes to which they are attached, the direction being generally from within 

 outward. In man and in the warm-blooded animals generally, the ciliated 

 or vibratile epithelium is of the variety called columnar, conoidal or ,pris- 

 moidal. The cilia are attached to the thick ends of the cells, and they form 

 on the surface of the membrane a continuous sheet of vibrating processes. 

 In general structure the ciliary processes are entirely homogeneous, and they 

 gradually taper from their attachment to the cell to an extremity of excess- 

 ive tenuity. 



The presence of cilia has been demonstrated upon the following surfaces : 

 The respiratory passages, including the nasal fossae, the pituitary membrane, 

 the summit of the larynx, the bronchial tubes, the superior surface of the 

 velum palati and the Eustachian tubes ; the sinuses about the head ; the 

 lachrymal sac and the internal surface of the eyelids ; the genital passages of 

 the female, from the middle of the neck of the uterus to the fimbriated 

 extremities of the Fallopian tubes ; the ventricles of the brain. In these 

 situations, on each cell of conoidal epithelium are 

 six to twelve prolongations, about 2 g& o0 of an inch 

 (1 p,) in thickness at their base, and -g-gVo to ^>Vo 

 of an inch (5 to 6 /n) in length. Between the cilia 

 and the substance of the cell, there is usually a 

 thin, transparent disk. The appearance of the cilia 

 is represented in Fig. 143. When seen in situ, 

 they appear regularly disposed upon the surface, 

 are of nearly equal length and are generally slight- 

 ly inclined in the direction of the opening of the 

 cavity lined by the membrane. 



When the ciliary movements are seen in a large number of cells in situ, 

 the appearance is well illustrated by the comparison by Henle to the undula- 

 tions of a field of wheat agitated by the wind. In watching this movement, 

 it is usually seen to gradually diminish in rapidity, until what at first ap- 

 peared simply as currents, produced by movements too rapid to be studied 

 in detail, become revealed as distinct undulations, in which the action of 

 individual cilia can be readily studied. Several kinds of movement have 

 been described, but the most common is a bending of the cilia, simultaneously 

 or in regular succession, in one direction, followed by an undulating return 

 31 



