THE TISSUES 539 



place, so that in one tissue the cells manifest special pro- 

 perties and carry on a special work ; in another they have 

 other properties, and other work ; and so on. 



5. The Chief Tissues of the Body. -The principal 

 tissues into which the undifferentiated cells of the embryo 

 become differentiated, and which are variously built up 

 into the organs and parts of the adult body, may be 

 arranged as follows. 



(i) The most important tis.sues are the musctllar and 

 nervous tissues, for it is by these that the active life of 

 the individual is carried on. 



(ii) Next come the epithelial tissues, which, on the 

 one hand, afford a covering for the surface of the body as 

 well as a lining for the various internal cavities of the 

 body : and, on the other hand, carry on a great deal of 

 the chemical work of the body, inasmuch as they form 

 the essential part of the various glandular organs of the 

 body. 



(iii) The remaining principal tissues of the body, 

 namely the so-called connective tissue, cartila- 

 ginous tissue, and osseous or bony tissue, form a 

 group by themselves, being all three similar in their 

 fundamental structure, and all three being, for the most 

 part, of use to the body for their passive rather than 

 for their active qualities. They chiefly serve to support 

 and connect the other tissues. 



These principal or fundamental tissues are often 

 arranged together to form more complex parts of the 

 body, which are sometimes spoken of, though in a different 

 sense, as tissues. Thus various forms of connective 

 tissue are built up with some muscular tissue and nervous 

 tissue, to form the blood-vessels of the body (see Lesson 

 II.), which are sometimes spoken of as " vascular-tissue." 

 So again, a certain kind of epithelial tissue, known as 

 " epidermis," together with connective tissue, blood-vessels 

 and nerves, forms the skin or tegumentary tissue : a 

 similar combination of epithelium with other tissues 



