16 NORMAL HISTOLOGY. 



THE ELEMENTARY TISSUES. 



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

 their structure, into four groups of elementary tissues the epithelial, the 

 connective, the muscular and the nervous. By the association and modifica- 

 tion of two or more of these tissues the organs are made up and acquire the 

 distinctive characteristics demanded by their function. A fifth group the 

 vascular tissues, including the blood-vessels and lymphatics with the con- 

 tained blood and lymph is sometimes added in view of the usual occurrence 

 of these tissues as constituents of organs. Since, however, the vascular tis- 

 sues are genetically related closely with the connective tissues and, where 

 highly specialized, are themselves composite in structure, it seems more 

 appropriate that they be not regarded as an independent group. 



THE EPITHELIAL TISSUES. 



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

 (epidermis) covering the exterior of the body and the epithelium lining the 

 digestive tube. Secondarily, they constitute the derivations of the epider- 

 mis, as hairs, nails and glands of the skin, and the lining of the ducts and 

 compartments of the glands connected with the digestive canal, as well as 

 the lining of the respiratory tract, which originates as an evagination from 

 the gut-tube. Further, epithelium forms the lining of the genito- urinary 

 tract. 



The primary purpose of the epithelium being to protect the delicate vas- 

 cular and nervous structures lying within the subjacent connective tissue of 

 the skin and of the mucous membranes, the epithelial cells are arranged as a 

 continuous sheet, the individual elements being united by a very small amount 

 of intercellular or cement substance. 



Epithelium is devoid of blood-vessels, the necessary nutrition of the 

 tissue being maintained by the absorption of nutritive juices which pass to 

 the cells by way of the minute clefts within the intercellular substance. The 

 distribution of nerve-fibres within epithelium ordinarily is scanty, although 

 in localities possessing a high degree of sensibility, as the tactile surfaces or 

 the cornea, the terminal nerve-nlaments may lie between the epithelial ele- 

 ments. Frequently the epithelium is separated from the connective tissue 

 upon which it rests by a delicate basement membrane or membrana propria. 

 The latter usually appears as a linear subepithelial boundary, being often 

 particularly well marked beneath the epithelium of glands. 



Based on the predominating form of the component cells, the epithelial 

 tissues are divided into two chief groups, squamous and columnar, each of 

 which is subdivided into simple and stratified, according to the presence of 

 a single or several layers of cells respectively. Modified epithelium includes 

 cells which exhibit adaptation and specialization to meet particular uses; 

 such are the ciliated, pigmented and glandular epithelia. Highly differenti- 

 ated neuro-epithelium occurs in the perceptive apparatus concerned in the 

 special senses, the gustatory cells of the taste-buds found on the tongue, the 

 rod- and cone-cells of the retina and the auditory cells of Corti's organ being 

 familiar examples. 



Squamous Epithelium. Where this variety of epithelial tissues 

 occurs as a single layer, it consists of flattened polyhedral nucleated plates 

 which, viewed from the surface, form a more or less regular mosaic. Hence 



