THE STRUCTURE OF THE ELEMENTARY TISSUES. 



epididymis; in the female (c.) commencing about the middle of the 

 neck of the uterus, and extending 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 epithe- 

 lium in the child, but in the adult this epithelium is limited to the 

 central canal of the cord. In the embryo the pharynx, oesophagus, and 

 part of the stomach may also be lined with ciliated cells, (e.) The ex- 

 cretory ducts of certain small glands in different localities, (f.) In 

 certain animals, especially the lower vertebrates, ciliated cells line the 

 beginning of the tubes of the kidneys. 



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

 variety of epithelium; they vary a good deal in size in different classes 



Fig. 29. 



Fig. 30. 



Fig. 29. Spheroidal ciliated cells from the mouth of the frog, x 300 diameters. (Sharpey.) 

 Fig. 30. Ciliated epithelium from the human trachea, a, Large, fully formed cell. 6, Shorter 

 cell; c, developing cells with more than one nucleus. (Cadiat.) 



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, in the 

 human trachea measuring j-g-J^ to -g-^nr of an inch, but nearly ten times 

 the length in the cells of the epididymis. 



The cilia themselves are fine rounded or flattened processes, appar- 

 ently homogeneous, pointed toward their free extremities. According 

 to some observers these processes are connected through intervening 

 knob-like junctions with longitudinal fibres which pass to the other 

 end of the cell, but which are not connected with the nucleus. 



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 in a drop of 

 0.6 per cent solution of common salt (normal saline solution), 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 



