20 INTRODUCTION 



purpose, and the whole body becomes a mass of many 

 different tissues, each having its definite and special char- 

 acteristics and structure. This process is known as dif- 

 ferentiation of the tissues, and is accompanied by what is 

 called physiological division of labor. One tissue is better 

 adapted to the performance of a certain office in the body 

 than are others, and that special work is given it to do, so 

 that the work of carrying on the operations of the body is 

 divided up among the tissues. 



12. Organs. A living being is often called an organism. 

 In order to secure the greatest efficiency in their labors, 

 the various tissues are built up into a multitude of mecha- 

 nisms called organs; for example, the eye, the hand, the 

 liver. Several different kinds of tissue often enter into 

 the structure of a single organ, and the same sort of tissue 

 appears in many different organs. 



A number of organs so arranged and related to one 

 another as to cooperate in carrying on a special process 

 or series of processes, is called a system, as the diges- 

 tive system, or the nervous system. 



13. Classification of Tissues. Tissues are variously clas- 

 sified by different authors, but one broad distinction may 

 be noted which divides them into two great classes : 



(1) The tissues which have to do with the setting free of 

 energy. These are the muscular and nervous tissues. 



(2) The tissues which have to do with renewing the sub- 

 stances and restoring the power of responding to stimulus. 

 In this second class are grouped all the remaining parts 

 of the body, which include tissues differing widely from 

 one another from the solid, bony tissue of the skeleton, 

 and the still harder enamel which covers the teeth, to the 

 soft substance composing the brain, the elastic fat which 

 rounds out the figure, and the fluid which we call blood. 



