78 ' PHYSIOLOGY OF THE DOMEST.IC ANIMALS. 



shrivel when desiccated, and their appearances then undergo the same 

 changes as will be described under the alterations of protoplasm. The 

 solutions which coagulate albuminoid bodies also coagulate the ciliate 

 prolongations of cells. Caustic alkalies and most of the concentrated 

 acids dissolve them. In fact, cilia behave to all reagents in a very 

 similar manner to protoplasm. All cilia are invariably connected with 

 a protoplasmic base, and are never on firm membranes. Therefore, 

 when cilia are found in the higher animals on epithelial membranes the 

 free surface of the cell possesses no membrane, but the protoplasmic cell- 

 contents, in a manner similar to that which is found in the epithelial cells 

 of the villi of the small intestine, is . somewhat condensed, apparently 

 non-contractile, homogeneous, or striated, and not capable of imbibition. 

 Such a surface might therefore be described as a protoplasmic cuticle. 



Cilia pass through this condensed layer of protoplasm to be directly 

 in contact with the protoplasmic contents of the cell below. On each cell 

 of a ciliated epithelial membrane, from ten to twenty such cilia will be 

 distributed over the external surface. In lower forms of animals, as in 

 the spermatozoa of all vertebrates, ciliated cells may possess but a single 

 cilium, as seen in many of the unicellular algae and flagellata. 



Ciliated epithelial cells are always cylindrical in shape and are 

 nucleated. When a portion of ciliated membrane such as that obtained 

 from the mouth or nasal pharynx of the frog, or from the nasal chamber 

 of almost any animal, is placed under the microscope, the thread-like 

 prolongations of these cells will be found to be in constant motion, by 

 which the cells of one locality make a rapid bending motion in one 

 direction, and then more slowly bend themselves back to their original 

 position. The amplitude of these oscillations varies greatly with the 

 character of the cell and certain external conditions, but on all cylin- 

 drical epithelial cells taken from the same localitjr is about equal, and, 

 although the bending may be as much as 90°, it usually varies from 

 about 20° to 50°. The rapidity of oscillation of each cilium may be 

 about six or eight in the second, although under certain circumstances 

 it may be considerably higher, since it is influenced by a number of 

 external conditions, such as temperature, amount of water contained by 

 imbibition, etc. 



The mechanical force exerted by this pendulum-like motion of the 

 cilia is very considerable. In cells which, like the spermatozoa, are 

 supplied with a single cilium (Fig. 52), the screw-like motion of this 

 appendage is sufficient to produce rapid motion of the entire organism. 

 In the case of ciliary membranes, the vibration, having a greater intensity 

 in one direction than in another, is sufficient to produce forward motion 

 of light bodies brought in contact with them. Thus, if the mucous mem- 

 brane is dissected from the pharynx of the frog and fastened by pins on 



