20 



HANDBOOK OF PHYSIOLOGY. 



C. Stratified. 



A. Simple. Squamous Epithelium (Fig. 13). Arranged as a single 

 layer, this form of epithelium is found as (a) the pigmentary layer of 

 the retina, and forms the lining of (#) the interior of the serous and 

 and synovial sacs, (c) the alveoli of the lungs, and (d) of the heart, blood - 

 and lymph-vessels. It consists of cells, which are flattened and scaly, 

 with a more or less irregular outline. 



J^ 7 



FIG. 13. Squamous epithelium scales from 

 the inside of the mouth, x 260. (Henle.) 



FIG. 14. Pigment cells from the retina. A, 

 cells still cohering, seen on their surface; a, 

 nucleus indistinctly seen. In the other cells 

 the nucleus is concealed by the pigment gra- 

 nules. B, two cells seen in profile ; a, the 

 outer or posterior part containing scarcely 

 any pigment. X 370. (Henle.) 



In the pigment cells of the retina, there is a deposit of pigment in 

 the cell-substance. This pigment consists of minute molecules of mela- 

 nin, imbedded in the cell-substance and almost concealing the nucleus, 

 which is itself transparent (Fig. 14). 



In white rabbits and other albino animals, in which the pigment of 

 the eye is absent, this layer is found to consist of colorless pavement 

 epithelial cells. 



The squamous epithelium which is found as a single layer lining the 

 alveoli of the lungs, the serous membranes, and the interior of blood- 

 and lymphatic-vessels, is generally called by a distinct name Endo- 

 thelium. 



The presence of endothelium may bo demonstrated by staining the 

 part lined by it with silver nitrate. 



When a small portion of a perfectly fresh serous membrane for ex- 

 ample, as the mesentery or omentum (Fig. 15), is immersed for a few 

 minutes in a quarter per cent solution of silver nitrate, washed with 

 distilled water and exposed to the action of light, the silver oxide is 

 precipitated in the intercellular cement substance and the endothelial 

 cells are thus mapped out by fine dark and generally sinous lines of ex- 

 treme delicacy. The cells vary in size and shape, and are as a rule 

 irregular in outline; those lining the interior of blood-vessels and lym- 

 phatics being spindle- shape with a very wavy outline. They inclose a 

 clear, oval nucleus, which, when the cell is viewed in profile, is seen to 

 project from its surface. The nuclei are not however evident unless the 

 tissue which has been already stained in silver nitrate, is placed in an- 



