OEGANIZATION. 33 



which secures the flexibility .of the organs so essential to 

 animal life. Every part of the human bbdy, for example, 

 is moist: even the hairs, nails, and cuticle contain water. 

 The contents as well as the shape of the cells are usually 

 modified according to the tissue which they form : thus, we 

 iind cells containing earthy matter, iron, fat, mucus, etc. 



In plants, the cell generally retains the characters of the 

 cell ; but in animals (after the embryonic period) the cell 

 usually undergoes such modifications that the cellular form 

 disappears. The cells are connected together or enveloped 

 by an intercellular substance (blastema), which may be wa- 

 tery, soft, and gelatinous, firmer and tenacious, still more 

 solid and hyaline, or hard and opaque. In the fluids of the 

 body, as the blood, the cells are separate ; i. e., the blastema 

 is fluid. But in the solid tissues the cells coalesce, being 

 simply connected, as in the epidermis, or united into fibres 

 and tubes. 



In the lowest forms of life, and in all the higher animals 

 in their earliest embryonic state, the cells of whic'u they are 

 composed are not transformed into difiEerentiated tissues: 

 definite tissues make their first appearance in the Sponges, 

 and they difiEer from one another more and more widely as 

 we ascend the scale of being. In other words, the bodies of 

 the lower and the immature animals are more uniform in 

 composition than the higher or adult forms. In the Verte- 

 brates only are all the following tissues found represented : 



(1) Epithelial Tissue. — This is the simplest form of cellu- 

 lar structure. It covers all the free surfaces of the body, 

 internal and external, so that an animal may be said to be 

 contained between the walls of a double bag. That which 

 is internal, lining the mouth, windpipe, lungs, blood-ves- 

 sels, gullet, stomach, intestines — in fact, every cavity and 

 canal — is called epithelium. It is a very delicate skin, 

 formed of flat or cylindrical cells, and in some parts (as in 

 the wind-pipe of air-breathing animals, and along the gills 



3 



