TISSUES AND ORGANS 149 



1. Epithelial tissue consists of thin sheets of cells which 

 usually cover surfaces. The external covering of meta- 

 zoans consists of epithelial tissues and their products; the 

 linings of various internal cavities and the membranes which 

 connect different organs are usually epithelial. 



2. Muscular tissue has specialized by developing its 

 power of contractility. The simplest muscular tissues 

 are made up of elongated single cells, but the more elabor- 

 ate types, like the striated muscles of arthropods and 

 vertebrates, are extremely complex and have various special 

 adaptations for making them contract more effectively. 

 There is one point to remember in regard to muscular 

 tissue it can only do work when contracting. 



3. Nervous tissue is composed of cells which have greatly 

 developed their powers of conducting and modifying im- 

 pulses. Most nerve cells have long fibers leading out from 

 the cell-body and these carry nervous impulses from place 

 to. place. Ganglia are places in nervous tissues where 

 there are more cells then fibers, and they serve as " relay 

 stations" for modifying or originating impulses. Nerves 

 are portions of nervous tissue which contain few or no cell- 

 bodies, but have many fibers. 



4. Sustentative, or Connective tissue, is quite variable but 

 is always characterized by the presence of a large amount of 

 intercellular substance. This material between the cells 

 may be bone (in osseous tissue), fibers (fibrous tissue), 

 cartilage (cartilaginous tissue), plasma (blood), or other 

 matter. 



In the bodies of most many-celled animals division of 

 labor has been carried beyond the formation of tissues. 

 Usually organs are formed by grouping various tissues to- 

 gether for carrying on particular labors. The heart, for 

 example, is an organ for pumping blood, and is composed 

 of muscular, epithelial, fibrous and nervous tissue. The 

 hand is an organ for prehension and offense; it is made up 

 of bone, cartilage, muscle, nerve, epithelium, etc. Organs 

 in the more complex animals are in turn grouped into 



