THE STRUCTURE OF THE ELEMENTARY TISSUES. 19 



first two of the great groups of tissues the Epithelial and Connective 

 will be described; while the others will be appropriately considered in 

 the chapters treating of their physiology. 



Epithelium. 



The term epithelium is applied to the cells covering the skin, the 

 mucous and serous membranes,, and to those forming a lining to other 

 parts of the body as well as entering into the formation of glands. For 

 example: 



Epithelium clothes (1) the exterior surface of the body, forming the 

 epidermis with its appendages nails and hairs; becoming continuous at 

 the chief orifices of the body nose, mouth, anus, and urethra with the 

 (2) epithelium which lines the whole length of the (3) respiratory, ali- 

 mentary and genito-urinary tracts, together with the ducts of their 

 various glands. Epithelium also lines the cavities of (4) the brain, and 

 the central canal of the spinal cord, (5) the serous aud synovial mem- 

 branes, and (6) the interior of all blood-vessels and lymphatics. 



Epithelial cells possess an intracellular and an intranuclear network 

 (p. 17). They are held together by a clear, albuminous, cement sub- 

 stance. The viscid semi-fluid consistency, both of cells and intercellular 

 substance, permits such changes of shape and arrangement in the indi- 

 vidual 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, but in some varieties minute 

 channels exist between the cells of certain layers through which they 

 may be supplied with nourishment 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 according 

 to the shape of the cells composing it. 



A. Simple. (1.) Squamous, scaly, pavement or tesselated; 



(2.) Spheroidal or glandular; 



(3.) Columnar, cylindrical, conical or goblet-shaped; 



(4.) Ciliated. 



B. Transitional. 



