CHAPTER XXI 



ORGANS AND SYSTEMS 



In the bodies of all Metazoa, except the lowest, tissues become 

 associated together in such a fashion that several contribute to the per- 

 formance of some function which belongs to the association as a whole. 



123. Definitions. — An organ is a part of the body formed by an 

 association of tissues all of which contribute to the performance of some 

 function or functions. Many organs in the higher animals include repre- 

 sentatives of all of the four different types of tissues. For instance, the 

 heart is covered and lined with epithelium; the greater part of its wall is 

 made up of muscular tissue; fibrous connective tissues serve to connect 

 other tissues and give support; and nervous tissues receive the impulses 

 from the central nervous system, coordinate them, and distribute them 

 to the heart muscles. 



Not only are tissues associated in the body to form organs, but organs 

 are associated to form systems. A system is a group of organs which 

 collectively perform certain related functions. Thus the body might 

 be conceived of as being built up by adding cells to cells to form 

 tissues, tissues to tissues to form organs, organs to organs to form systems, 

 and systems to systems to form the whole; or it could be analyzed in 

 terms first of systems, then of organs, then of tissues, and finally of cells. 



124. Systems. — Nine systems are recognized in higher animals. A 

 list of these with the most prominent functions follows: 



1. Tegumeiitary System. — Protection, temperature regulation, elimi- 

 nation of a small amount of liquid waste, and external support. 



2. Digestive System. — The ingestion, digestion, and absorption of food, 

 the secretion of digestive ferments, egestion, and elimination of a small 

 amount of liquid waste. 



3. Circulatory System. — The transportation of food, oxygen, and the 

 excretions of the body as well as the carrying about of certain internal 

 secretions; also internal respiration. 



4. Respiratory System. — The taking in of oxygen and giving off of 

 carbon dioxide, or external respiration. 



5. Excretory System. — The elimination of most of the hquid waste 

 products derived from metabohsm (this would be more appropriately 

 named the eliminative system, but the name excretory has been univer- 

 sally used). 



6. Skeletal System. — Protection and support. 



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