CILIARY MOVEMENTS. 523 



nature and cause of the movements of homogeneous contractile substance ; and it must 

 be excessively difficult, if not impossible, to observe directly the effects of different stim- 

 uli, in the manner in which we study the movements of muscles. As far as we can 

 judge, they are analogous to the ciliary movements, the cause of which is equally obscure. 



Ciliary Movements. The epithelium covering certain of the mucous membranes is pro- 

 vided with little hair-like processes upon the free portion 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 membranes to which they are attached, the direction being gener- 

 ally from within outward. In many of the infusoria, the ciliary motion serves as a means 

 of progression, effects the introduction of nutriment into the alimentary canal, and, indeed, 

 is almost the sole agent in the performance of the functions involving movement. Even 

 in higher classes, as the mollusca, the movements of the cilia are of great importance. 

 In man and in the warm-blooded animals generally, the ciliated or vibratile epithelium is 

 of the variety called columnar, conoidal, or prismoidal. 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. 



It is unnecessary to describe in detail the size and form of the cells provided with 

 cilia, as their variations in different situations have been and will be considered in con- 

 nection with the physiological anatomy of different parts. In general structure, the ciliary 

 processes are entirely homogeneous, and they gradually taper from their attachment to 

 the cell to an extremity of excessive tenuity. Although anatomists, from time to time, 

 have described striaa at the bases of the cilia and have attempted to explain their mo- 

 tion by a kind of muscular action, no well-defined structure has ever been actually 

 demonstrated in their substance. 



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

 tory 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 extremities of the Fallopian tubes ; and the ventricles of the brain. They prob- 

 ably exist, also, at the neck of the capsule of Muller, in the cortical substance of the kid- 

 ney. In these situations, to each cell of 

 conoidal epithelium are attached from six to 

 twelve prolongations, about 3-5^-5^ f an ^ nc ^ 

 in thickness at their base, and from -g-^Vfr to 

 a-gVff f an mcn in length. The appearance 

 of the cilia in detached cells is represented 

 in Fig. 147. When seen in situ, they ap- 

 pear regularly disposed upon the surface, are 

 of nearly equal length, and are generally 

 slightly inclined in the direction of the open- 

 ing of the cavity lined by the membrane. 



The ciliary motion is one of the most 

 beautiful physiological demonstrations that 

 can be made with the microscope. By scrap- 

 ing the roof of the mouth of a living frog, 

 the mucous membranes of the respiratory 

 passages in a warm - blooded animal just 

 killed, the beard of the oyster or clam, and FlGt 

 placing the preparation, moistened with a 

 little serum, under a magnifying power of about two hundred and fifty diameters, the 

 currents produced in the liquid will be strikingly exhibited. The movements may be 



