22 



INTRODUCTION 



the one case by contraction, in the other by some 

 change not yet understood, giving rise to what is called a 

 " nervous impulse," and in the act they develop energy 

 by the breaking down of their own substance. Unless 

 that substance is renewed the tissue will cease to respond to 

 stimulus, will die. The remaining tissues of the body, 

 therefore, are engaged in one way or another in preparing 

 the needful food, in conveying it to these " master tissues," 

 in taking up the waste substances produced in the evolu- 

 tion of energy and preparing them for removal from the 

 body, or in furnishing mechanical support to the body and 

 its various parts. In these processes are involved all the 

 parts which are concerned in digestion, respiration, circu- 

 lation, and excretion. This varied and complex series of 

 operations implies a vast array of muscular movements, 

 and all are governed by the nervous system under the 

 action of its varied stimuli. 



16. Epithelial Tissues. The free surfaces of the body, 

 both within and without, are covered with a tissue called 

 epithelium. Simple epithelium is composed of but one 



layer of cells, ar- 

 ranged like flat pav- 

 ing stones and fitted 

 together with a very 

 little cementing mate- 

 rial (Fig. 9, A). The 

 epithelium of this va- 

 riety found lining the 

 interior of the blood 

 vessels, and some other surfaces which are not exposed 

 to the outer air, is called endothelium. 



The cubical, spheroidal, and columnar epithelial tissues 

 are named from the shape of their cells. In ciliated 



Fig. 9. Columnar epithelium. 

 A simple. B stratified. 



