Chapter 4 



CELLULAR ORGANIZATION OF PLANTS AND 

 ANIMALS— ANIMAL AND PLANT TISSUES 



I. ANIMAL TISSUES 



A tissue is a group of similar cells differentiated so as to perform cer- 

 tain functions. According to the cell principle, all living organisms are 

 composed of cells. Consequently, all tissues and organs of an organism 

 are composed of cells. Upon casual and hurried observation all tissues 

 may appear to be made in the same manner, but scientific, microscopic 

 examinations show that the various tissues differ in structure and func- 

 tions. In order that the functions of an organism, or its parts, may be 

 properly understood, it is necessary to be familiar with the cellular struc- 

 ture of its tissues. In other words, a knowledge of anatomy must precede 

 physiology. The characteristics of the more important animal tissues 

 will be given in table form in order that they may be compared and con- 

 trasted more easily. 



Kinds of Animal Tissues 



EPITHELIAL 

 (ep i -the' li al) 

 (Gr. epi, upon; 

 thele, nipple) 

 (Fig. 11) 



A layer of tissue composed of flat, cuboidal, or column- 

 shaped cells, depending on the type of epithelium; there 

 is a minimum of intercellular space between cells; they 

 compactly cover the surface and line the cavities of the 

 body which usually lead to the outside; they are not sup- 

 plied with blood vessels but must absorb nourishment 

 from the blood and lymph as they pass the cells. 



Functions: Protective, absorptive, secretive, excretive, 

 sensory. 



B. CONNECTIVE 

 (SUPPOR- 

 TIVE) 



(ko -nek' tiv) 

 (L. cum, to- 

 gether; nectere, 

 to bind) (Figs. 

 12 and 13) 



Fibers are usually present, and much nonliving material 

 (fibers, plates, masses, etc.) is produced by the cells; 

 there is a maximum of intercellular space; they are 

 common in most parts of the body; they all arise em- 

 bryologically from the same source (mesenchyme cells). 



Functions: bind body parts together; some kinds form 

 semirigid, or rigid, structures for protection and attach- 

 ment of other tissues and organs. 



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