THE STRUCTURE OF THE ELEMENTARY TISSUES. 25 



open mouth of the globet, leaving the nucleus surrounded by the re- 

 mains of the protoplasm in its narrow stem. 



This transformation is a normal process which is continually going 

 on during life, the discharged cell-contents contributing to form mucus, 

 the cells being supposed in many cases to recover their original shape. 

 It is an example of secretion. 



4. Ciliated cells are generally cylindrical (Fig. 23, B), but may be 

 spheroidal or even almost squamous in shape (Fig. 23, A). 



This form of epithelium lines (a.) the whole of the respiratory 

 tract from the larynx, except over the vocal cords, to the finest sub- 

 divisions of the bronchi, also the lower parts of the nasal passages, the 

 nasal duct, and the lachrymal sac. In, part of j this tract, however, 

 the epithelium is, in several layers, of which only the most superficial 

 is ciliated, so that it should' more accurately -be termed transitional 

 (p. 26) pr stratified, (b. ), some portions of the generative apparatus 

 in the male, viz., lining the " vasa efferentia" of the testicle, and their 

 prolongations as far as the lower end of the epididymis; in the female 

 (c.) commencing about the middle of the neck of the uterus, and ex- 

 tending throughout the uterus and Fallopian tubes to their fimbriated 

 extremities, and even for a short distance on the peritoneal surface of 

 the latter, (d. ) The ventricles of the brain and the central canal of 

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

 the adult this epithelium is limited to the central canal of the cord. 



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

 variety of epithelium, vary a good deal in size in different classes of ani- 

 mals, 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 

 living ciliated epithelium, e. g., from the gill of a mussel, or oyster, or 

 from the mouth of the frog, or from a scraping from a polypus from 

 the human nose, 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 general impression given to the 

 eye of the observer is very similar to that produced by waves in a field 

 of corn, or swifty running and rippling water, 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, towards its external orifice. 



Ciliary Motion, Ciliary, which is closely allied to amoeboid and 

 muscular motion, is alike independent of the will, of the direct influence 

 of the nervous system, and of muscular contraction. It continues for 

 several hoars after death or removal from the body, provided the portion 

 of tissue under examination be kept moist. Its independence of the ner- 



