66 ZOOLOGY 



and is of great advantage. We have described these structure- 

 groups as tissues (see Chapter V) . 



90. Organs. The tissues which have been described are 

 never independent, but are associated with each other in the per- 

 formance of a common function, to form an organ. In each 

 organ there is usually a principal tissue which determines its 

 function (as muscular tissue in muscle, or the glandular tissue 

 in glands), and one or more accessory tissues for support or con- 

 trol (as connective or nervous tissue in the organs mentioned) . 

 To accomplish some of the activities, in the higher animals 

 especially, several organs of a similar kind must work together. 

 These are sometimes spoken of collectively as systems of organs, 

 e.g., digestive system, circulatory system, and the like. 



91. Classification of the Systems of Organs and Func- 

 tions. The work that needs to be done by an organism may be 

 considered under the following heads: (i) metabolism in- 

 cluding digestion, circulation, assimilation, respiration, and 

 excretion; (2) protection and physical support; (3) growth; 

 (4) reproduction; (5) movement; (6) sensation. Eight sys- 

 tems of organs may be distinguished by which this work is done. 

 They are (i) the digestive; (2) circulatory; (3) respiratory; (4) 

 excretory; (5) skeletal and integumentary; (6) reproductive; 

 (7) muscular, and (8) nervous. 



92. Metabolism (Nutrition). Metabolism embraces two 

 sets of processes, (i) constructive or anabolic, known as assimi- 

 lation, and (2) destructive or katabolic. By constructive we 

 mean all the building-up processes in the organism which 

 result in the storing of food and energy, in growth, repair, and 

 reproduction. We class as destructive all those processes by 

 which the complex cell substances are broken down or rear- 

 ranged, and energy set free, leading to change of temperature, to 

 nervous or muscular action, to secretion and excretion. In the 

 higher animals the nutritive process is a very complicated one 

 and demands the cooperation of numerous organs. It embraces 

 the ingestion or taking in of food; the digestion of food; its 

 absorption from the digestive tract into the body fluids blood 



