THE ELEMENTARY TISSUES. 



The various parts and organs of the complex body may be resolved, in their 

 morphological constitution, into a few component or elementary tissues, of which 

 there are four principal groups, — the epithelial, the connective, the muscular, and 

 the nervous tissues. The first two of these may be discussed at this place ; the re- 

 maining groups, the muscular and the nervous tissues, are considered most advan- 

 tageously in connection with the muscular and nervous systems to which they are 

 directly related and under which sections they will be found. 



THE EPITHELIAL TISSUES. 



The epithelial tissues include, primarily, the integumentary sheet of protecting 

 cells covering the exterior of the body and the epithelium lining the digestive tube. 

 Secondarily, they embrace the epithelial derivatives of the epidermis, such as the 

 nails, hairs, and glands of the skin and its extensions, and the epithelial lining of 

 the ducts and compartments of the glands formed as outgrowths from the primi- 

 tive gut-tube, as well as the epithelium clothing the respiratory tract which originates 

 as an evagination from the digestive canal. 



An apparent exception to the usual origin of the epithelial tissues from either 

 the ectoblast or the entoblast is presented by the lining of the genito-urinary tract, 

 since all the epithelium occurring in connection with these organs, as far as the 

 bladder, is of mesoblastic origin, and hence genetically related closely with the 

 extensive mesoblastic group of tissues. It is to be noted in this connection that the 

 epithelium of the bladder and of a part of the urethra is derived from outgrowths of 

 the primary gut, and therefore is entoblastic in origin. 



The primary purpose of epithelium being protection of the more delicate 

 vascular and nervous structures lying within the subjacent connective tissue of the 

 integument or of the mucous membrane, the protecting cells are arranged as a con- 

 tinuous sheet, the individual elements being united by a small amount of inter- 

 cellular substance. 



Epithelium contains no blood-vessels, the necessary nutrition of the tissue being 

 maintained by the absorption of the nutritive juices which pass to the cells by way 

 of the minute clefts within the intercellular substance. Likewise, the supply of 

 nerve-fibres within epithelium ordinarily is scanty, although in certain localities 

 possessing a high degree of sensibility, as the cornea or tactile surfaces, the termi- 

 nations of the nerves may lie between the epithelial elements. 



The epithelial tissues are frequently separated from the subjacent connective 

 tissue by a delicate basement membrane, or membrana pi'opria ; the latter, which may 

 be regarded as a derivative or modification of the connective tissue, usually appears 

 as a delicate subepithelial boundary, being particularly well marked beneath the 

 epithelium of glands. 



According to the predominating form of the component cells, the epithelial 

 tissues are best divided into two chief groups, — squamous and columnar, — with sub- 

 divisions as shown in the following table : 



VARIETIES OF EPITHELIUM. 



I. — Squamous : 



a. Simple, — consisting of a single layer. 



b. Stratified, — consisting of several layers. 

 II. — Columnar : 



a. Simple, — consisting of a single layer. 



b. Stratified, — consisting of several layers. 

 III. — Modified : 



a. Ciliated, b. Goblet, c. Pigmented. 

 IV.— Specialized : 



a. Glandular epithelium, b. Neuro-epithelium. 



67 



