CHAPTER VII. 

 THE ORGANS. 



In the lowest order of animals, the protozoa, the single cell, 

 which constitutes the whole individual, performs all the functions 

 necessary to the life of the animal; but in the higher multicellular 

 animals, the metazoa, those functions are distributed among a num- 

 ber of different but definite structures, called organs, each of which 

 is composed of certain of the elementary tissues arranged according 

 to a definite and characteristic plan peculiar to the organ. 



Within each organ certain of the elementary tissues are charged 

 with the immediate performance of the function assigned to that 

 organ. These tissues are collectively termed the parenchyma of 

 the organ. Thus, for example, the epithelium entering into the 

 composition of the liver and doing the work peculiar to that organ, 

 constitutes its parenchyma. The parenchyma of the heart is its 

 muscular tissue, through the activity of which it is enabled to con- 

 tract upon its contents. 



Functionally ancillary to its parenchyma, each organ possesses a 

 variety of elementary tissues, some of which belong to the connec- 

 tive-tissue group, which serve to hold the tissue-elements of the 

 parenchyma in position, to bring to them the nutrient fluids neces- 

 sary for their work, and to convey to them the nervous stimuli 

 which excite and control their functional activities. These sub- 

 sidiary tissues are collectively known as the interstitium of the 

 organ. For example, the fibrous tissue and the elementary tissues 

 forming the bloodvessels, lymphatics, and nerves of the liver, or of 

 the heart, form the interstitia of those organs. 



Two sets of structures entering into the formation of the inter- 

 stitia of the organs — namely, the nerves and the vessels, including 

 those which convey blood and those through which the lymph cir- 

 culates — have a similar general structure in all the organs, and are 

 connected with each other throughout the body, forming " systems." 

 These systems serve to bring the various parts of the body, so 

 diverse in structure and function and yet so interdependent upon 



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