MUCOUS MEMBRANES. EPITHELIUM. 



235 



form part of the boundary of one of the internal cavities (as in the plastic ope- 

 rations for the restoration of lips, eyelids/ &c.), it undergoes a gradual modifi- 

 cation in its characters, and comes, after a time, to present the appearance of an 

 ordinary Mucous membrane. 



230. For our knowledge of the constant existence of the Epithelium as an 

 integral constituent of the Mucous and Serous membranes, and for our apprecia- 

 tion of its important offices in the economy, we are entirely indebted to the 

 assistance afforded by the Microscope. It had long been known that the epider- 

 mic layer might be traced continuously from the lips to the mucous membrane 

 of the mouth, and thence down the oesophagus into the stomach ; and that, in 

 the strong muscular stomach or gizzard of the granivorous birds, it becomes 

 quite a firm horny lining. But it has been only since the application of the 

 Microscope to this investigation, that a continuous layer of cells has been traced, 

 not merely along the whole surface of the mucous membrane lining the alimen- 

 tary canal, but likewise along the free surfaces of all other Mucous Membranes, 

 with their prolongations into follicles and glands ; as well as on the Serous and 

 Synovial membranes, and the lining membrane of the heart, bloodvessels, and 

 absorbents. The forms presented by the Epithelial cells are various. The two 

 chief, however, are the tessellated, forming the " pavement-epithelium ;" and the 

 cylindrical forming the "cylinder-epithelium." The "pavement-epithelium" 

 covers the serous and synovial membranes, the lining membrane of the blood- 

 vessels, and the ultimate follicles or tubuli of most glandular structures con- 

 nected with the skin or mucous membranes, as also the mucous membranes them- 

 selves, where the cylinder-epithelium does not exist. The cells composing it are 

 usually flattened (Fig. 24, A), and sometimes so polygonal as to come into contact 

 with each other at their edges, like the pieces of a tessellated pavement (Fig. 13) ; 



Fig. 24. 



Separated Epithelium-cells, a, with nuclei, 

 b, and nucleoli, c, from mucous membrane of 

 the mouth. 



Pavement-Epithelium of the Mucous 

 Membrane of the smaller bronchial 

 tubes ; a, nuclei with double nucleoli. 



Fig. 25. 



but they sometimes retain their rounded or oval form, and are separated from 

 each other by considerable interstices (Fig. 24, B). This last form seems to be 

 the commonest, where the cells are most actively renewed, so that they have 

 not time (so to speak) to be developed 

 into a continuous stratum. The num- 

 ber of layers is commonly small ; and 

 usually there is only a single one. The 

 "cylinder-epithelium" is very differ- 

 ently constituted. Its component cells 

 are cylinders, which are arranged side 

 by side (Fig. 25); one extremity of 



each cylinder resting upon the base- 

 ment membrane, whilst the other forms 

 part f the free surface. The perfect 



Cylinder-Epithelium, from the intestinal villi of a 

 rabbit; a, a, membrane connecting their free surfaces, 

 rendered more distinct by the action of water. 



