7 



Ti 



issues 



In multicellular animals, cells live crowded together and constantly affected 

 by one another. Whether similar or different they cooperate closely in the 

 organization of the animal. Differentiation, the modification of certain parts 

 for certain functions, and cooperation are fundamental properties of their 

 structure and activity. The body of a flying bird and the body of the pilot of 

 an airplane are both great companies of cooperating cells. 



Cells are associated in groups, the tissues and organs, and these in turn in 

 systems. The study of groups of cells is histology or microscopic anatomy. 



Tissues, Organs, and Systems. Tissues are groups of similar cells with the 

 intercellular substances which they may produce. The substances may be of 

 hardly noticeable amount as in epithelium, or conspicuous as in bone, or fluid 

 as in blood. 



An organ is an association of tissues all of which cooperate toward the per- 

 formance of one or more particular functions. The heart is an organ that con- 

 sists largely of muscle; it is covered and lined with epithelium; nervous tissue 

 acts in the control of its pulsation; and connective tissue holds the other tissues 

 together. 



A system is a group of organs which collectively perform certain related 

 functions. The digestive system is concerned with intake of food, its prepara- 

 tion for absorption, and elimination of undigested waste substances. 



The animal body, like the plant, is built of groups of cells that form tissues, 

 of tissues that form organs, of organs forming systems, and of systems that 

 compose the whole body. 



The tissues are discussed in this chapter. The organs are included with their 

 respective systems. 



Classes of Tissue 



There are four types of tissues, grouped according to their structure and 

 function: epithelial, connective tissue including blood and supporting tissues, 

 muscular, and nervous tissue. 



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