26 HAND-BOOK OF PHYSIOLOGY. 



spinal cord are clothed with ciliated epithelium in the child, but in the 

 adult it is limited to the central canal of the cord. 



The Cilia, or fine hair-like processes which give the name to this va- 

 riety of epithelium, vary a good deal in size in different classes of animals, 

 being very much smaller in the higher than among the lower orders, in 

 which they sometimes exceed in length the cell itself. 



The number of cilia on any one cell ranges from ten to thirty, and 

 those attached to the same cell are often of different lengths. When liv- 

 ing ciliated epithelium, e.g.., the gill of a mussel, is examined under the 

 microscope, the cilia are seen to be in constant rapid motion; each cilium 

 being fixed at one end, and swinging or lashing to and fro. The gen- 

 eral impression given to the eye of the observer is very similar to that pro- 

 duced by waves in a field of corn, or swiftly running and rippling water, 



FIG. 21. A. Spheroidal ciliated cells from the mouth of the frog. X 300 diameters. (Sharpey.) 

 B. a. Ciliated columnar epithelium lining a bronchus, b. Branched connective-tissue corpuscles. 



(Klein and Noble Smith.) 



and the result of their movement is to produce a continuous current in 

 a definite direction, and this direction is invariably the same on the same 

 surface, being always, in the case of a cavity, toward its external orifice. 



5. Transitional Epithelium-. This term has been applied to cells 

 which are neither arranged in a single layer, as is the case with simple 

 epithelium, nor yet in many superimposed strata as in laminated; in other 

 words, the term is employed when epithelial cells are found in two, three, 

 or four superimposed layers. The upper layer may be either columnar, 

 ciliated, or squamous. When the upper layer is columnar or ciliated, the 

 second layer consists of smaller cells fitted into the inequalities of the 

 cells above them, as in the trachea (Fig. 21, B). The epithelium which 

 is met with lining the urinary bladder and ureters is, however, the tran- 

 sitional par excellence. In this variety there are two or three layers of 

 cells, the upper being more or less flattened according to the full or col- 

 lapsed condition of the organ, their under surface being marked with 

 one or more depressions, into which the heads of the next layer of club- 

 shaped cells fit. Between the lower and narrower parts of the second row 

 of cells, are fixed the irregular cells which constitute the third row, and 

 in like manner sometimes a fourth row (Fig. 22). It can be easily under- 

 stood, therefore, that if a scraping of the mucous membrane of the blad- 



