94 ANIMAL PHYSIOLOGY. [LESSON 28. 



absorbents ; the follicles of the mucous membrane, and all the glands ; 

 the trachea (windpipe) and all its ramifications, no less than the air- 

 cells of the lungs. 



469. Epithelial cells are various in figure, but resolve themselves 

 into two chief forms the tessellated or pavement epithelium, and the 

 cylindrical, or cylinder epithelium. 



Both these forms may be provided with vibratile cilia, and each 

 has a tendency to run into the other. 



470. The cells of the tessellated or pavement epithelium (Fig. 

 162) are flattened, and polygonal ; they are attached to each other 



like the elements of a tessellated pave- 

 ment, by the numerous angular sur- 

 faces each scale presents hence the 

 name bestowed on this particular form 

 of epithelium. It occasionally hap- 

 pens, however, that they retain their 

 original rounded or oval form, and in 

 this case they are found separated by 

 an interval of space from each other. 

 471. All epithelial cells, without 

 respect to form, are remarkable for 

 the possession of a distinct nucleated 



spot ; whence this nucleus is obtained is at this time a mystery ; it is 

 quite likely that they produce other epithelial cells, on the principle 

 of cell development, and this is all that is known in relation to the 

 subject. 



472. The cylinder-epithelium is composed of elongated cells, cy- 

 lindrical in their form ; they are placed side by side, in a uniform se- 

 ries, the lower portion (frequently attenuated) constituting the base, 

 and the upper, exposed portion, projecting into the free space of the 

 tissue to which it is attached. 



473. To see the well-formed cylindrical cell, a tissue should be 

 selected with a flat surface ; if it be convex, the lower ends of the 

 epithelial cells are always of less diameter than the upper portion 

 (attenuated), so that these epithelium scales much more resemble a 

 series of truncated cones, than cylinders ; this fact is well shown 

 in the epithelium covering the villi of the intestines, of which more 

 will be said in the proper place. 



474. The Cylinder epithelium is found covering the entire sur- 

 face of the alimentary canal, from the cardiac orifice of the stomach 

 to its termination ; it is also found in the ducts of the glands which 



