26 



EPITHELIUM. 



|"cii. ITT. 



Pavement Epithelium. 



This consists of a layer of flat cells, arranged like flat pavement- 

 stones accurately fitting together and united by a small amount 

 of cementing material. The structure of the cells and their 

 outlines may be best demonstrated by a process of double 



staining ; one stain, silver nitrate, 

 to show the cementing material, 

 and another, like logwood, to 

 show up the nuclei of the cells. 



A portion of the fresh tissue 

 is taken and immersed for a few 

 minutes in a i per cent, solution 

 of nitrate of silver ; it is taken 

 out, washed with distilled water, 

 and exposed in water or spirit to 

 sunlight. The silver forms a com- 

 pound with the cement, which 

 in the light is decomposed or re- 

 duced, leading to a fine deposit 

 of silver, showing as black or 

 brown lines between the cells, and 

 accurately defining their outlines. 

 The preparation may then be 

 immersed in some stain like log- 

 wood to bring out the nuclei, and 

 finally mounted in the usual way. 

 The de'ails of histological work 

 can only be properly learnt in a 

 practical class. 



Fig. 28 shows the appearance 

 presented in a preparation of 

 lung. In the alveoli or air-sacs 

 of the lung, pavement epithelium 

 of a typical kind is found form- 

 ing a lining membrane. 



Endothelium. Epithelium of similar appearance is found 

 lining the interior of the whole of the vascular system, heart, 

 arteries, capillaries, veins, and lymphatics, and in the adjuncts of 

 the circulatory system called the serous membranes (pericardium, 

 peritoneum, etc.). 



This epithelium is formed from the middle layer of the embryo, 

 the mesoblast ; most other epithelium is derived either from 



Fig. 32. Surface view of an artery from 

 the mesentery of a frog, ensheathed 

 in a peri-vascular lymphatic vessel. 

 a, The artery, with its circular mus- 

 cular coat (media) indicated by broad 

 transverse markings, with an indica- 

 tion of the adventitia outside. 

 I, Lymphatic vessel ; its wall is a 

 simple endothelial membrane. (Klein 

 and Noble Smith.) 



