TISSUES, ORGANS AND SYSTEMS 33 



epithelial tissue, of which the skin and lining of the 

 mouth and digestive tract furnish good examples. 



While an organ may be defined as any structure fitted 

 to perform a definite function, and thus includes the 

 nucleus, cilia, and other parts of a single cell, it involves, 

 in higher organisms a grouping of tissues. The arm, for 

 example, is an organ, and comprises all of the tissues just 

 mentioned. Then again several organs may be engaged 

 in related work and so form a system. The oesophagus, 

 stomach, intestine, liver, and pancreas are all organs 

 serving in the process of digestion, and in combination 

 constitute the digestive system. Other organs combine 

 to form the nervous, muscular, circulatory, and other 

 systems that will be described in some of the succeeding 

 chapters. 



