546 ELEMENTARY PHYSIOLOGY less 



The polygonal hepatic cells (see p. 212), are in ro;ility the 

 epithelium cells beloni^ing to the minute biliary ca,nals 

 passing between them. 



In the trachea and bronchi, the epithelium of the 

 mucous membrane consists again of a single layer of 

 cells, which are cylimlrical in form and ciliated. In 

 the ureter and bladder, on the other hand, the epithelium 

 consists of several layers of cells which are frequently 

 irregular in form. 



Lastly, the blood-vessels and lymphatic vessels and the 

 large serous cavities, such as the peritoneal and pleural 

 cavities, are lined by a peculiar epithelium, different in 

 origin from the epithelium of the skin and mucous mem- 

 branes. It consists of a single layer of flat, nucleated 

 plates cemented together at their edges. The form of 

 the plate or cell varies, being sometimes polygonal, 

 sometimes spindle-shaped, sometimes quite irregular (see 

 Fig. 29). 



10. The Structure of Cartilage.— A second group of 

 tissues, of which cartilage may be taken as the simplest 

 form and the type, differs from epithelium in a very 

 essential feature. In e{)ithelium, wherever it is found, the 

 cells are placed close together, and the amount of 

 material existing between the cells or intercellular material 

 is exceedingly small. In the group of tissues, however, 

 to which cartilage belongs, a very considerable quantity of 

 intercellular material is, as we shall see, developed 

 between the individual nucleated protoplasmic cells. 

 Hence the cells are, more or less, distinctly imbedded 

 in a substance different from themselves and called a 

 matrix. In epithelium, though the cells are sometimes 

 joined together by a cement material, this is never abun- 

 dant enough to deserve the name of matrix. 



(i) Hyaline Cartilage. — Characteristic specimens of this 

 tissue are to be found in the " sterno-costal cartilages," 

 which unite many of the ribs with the breastbone. A 

 thin but tough layer of vascular connective tissue invests, 



