THE STRUCTURE OF THE ELEMENTARY TISSUES. 29 



cells as are necessary if the epithelium is to maintain its integrity in 

 organs the area of whose free surface is so constantly changing, as the 

 stomach, lungs, etc. Thus, if there be but a single layer of cells, as in 

 the epithelium lining the air vesicles of the lungs, the stretching of this 

 membrane causes such a thinning out of the cells that they change 

 their shape from spheroidal or short columnar, to squamous, and vice 

 versa, when the membrane shrinks. 



Epithelial tissues are non-vascular, that is to say, do not contain 

 blood-vessels, but in some varieties minute channels exist between the 

 cells of certain layers through which they may be supplied with nour- 

 ishment from the subjacent blood-vessels. Nerve fibres are supplied to 

 the cells of many epithelia. 



Epithelial tissue is classified according as the cells composing it are 

 arranged in a single layer when it is simple, or in several layers when it 

 is called stratified or laminated, or in two or three layers occupying a 

 position between the other two forms, when it is termed transitional. 

 Of each form, when there are several varieties, they are named accord- 

 ing to the shape of the cells composing it. 



Classification of Epithelium. 



(a) Simple. (1.) Squamous, scaly, pavement, or tessellated; (2.) 

 Spheroidal or glandular: (3.) Columnar, cylindrical, conical or goblet- 

 shaped; (4.) Ciliated. 



(b) -Transitional. 



(c) Stratified. 



(a) Simple Epithelium. 



Squamous Epithelium. This form of epithelium is found arranged 

 as a single layer of flattened cells, as (a) the pigmentary layer of the 

 retina, and forms the lining of (b) the interior of the serous and syno- 

 vial 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. 



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

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

 colored substance, melanin, imbedded in the cell-substance and almost 

 concealing the nucleus, which is itself transparent. 



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 

 serous membranes, and the interior of blood- and lymphatic-vessels, is 

 generally called by a distinct name Endothelium. 



