A tissue such as any one of those described in the preceding chap- 

 ters scarcely exists in the pure state, that is alone by itself, in the adult 

 body. Two or more tissues are always associated and such an associa- 

 tion of two or more tissues for the purpose of performing a definite 

 function constitutes an organ. 



In practically all cases one of the tissues is connective tissue, the 

 main function of which is to form a supportive framework for the 

 more active specific tissue of the organ. The connective tissue is 

 often spoken of as the interstitial tissue, the specific tissue as the 

 parenchyma. In many cases the connective tissue forms a definite 

 covering or capsule. From the capsule strands of connective tissue 

 frequently extend down into the organ where they divide and sub- 

 divide to form its connective tissue framework. Sometimes the 

 subdivision of the organ by connective tissue is quite regular, macro- 

 scopic subdivisions being marked off by coarse connective tissue 

 septa, and these again being subdivided by finer septa. In such 

 case the former are known as lobes, the latter as lobules. 



But while an organ has been defined as consisting of two or more 

 tissues and while in general one of these tissues is the connective tissue 

 framework and the other the specific functioning tissue of the organ, 

 such a simple combination of two tissue does not actually exist, for all 

 organs are supplied with blood and lymph which are distributed to 

 them and through them by the blood and lymph vessels. There are 

 thus carried into the organs not only the blood and lymph themselves, 

 but also the tissues which compose the walls of the blood and lymph 

 vessels. Also every organ has its nerve supply; thus nerve tissue is 

 distributed through all organs. 



Many organs are hollow tubes and their study is the study of the 

 structure of the walls of the tube, such, e.g., are the stomach and 

 intestines, the heart and the blood-vessels, the trachea and bronchi. 

 Other organs not so apparently tubular still show their tubular struc- 

 ture on closer analysis, such, e.g., are the lungs and the duct glands. 



As an organ has a specific function and as this function is performed 

 mainly by the specific cells of the organ, these cells show variations 

 in structure dependent upon whether the organ is at work or at rest. 



Many adult organs are so complex that it is possible to understand 



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c 



