GENERAL STRUCTURE. 



The study of any vertebrate reveals the presence of 

 numerous organs, each of which is for the performance 

 of a particular function. Thus the heart is the organ for 

 the propulsion of the blood, the kidney for the elimination 

 of the nitrogenous waste. Several organs combined for a 

 common purpose constitute a system. The heart, with the 

 various vessels for conveying the blood, forms the circula- 

 tory system. The following eight systems are found in all 



FIG. 7. FLAT EPITHELIUM CELLS FIG. 8. INVOLUNTARY MUSCLE- 

 FROM THE MOUTH. X 150. CELLS. X 250. 



n, Nucleus of the cell. n, Nucleus of a cell. 



Mammalia: Osseous or bony, muscular, digestive, respira- 

 tory, vascular, excretory, reproductive, and nervous. 



The relative locations of the various systems are repre- 

 sented diagrammatically in Fig. 54. The organs have the 

 same arrangement throughout all the orders of mammals. 

 Moreover, the minute structure of the same organ is so 

 similar in the different species that in many cases even the 

 microscope will not enable one to tell from which of several 

 kinds of mammals the organ has been taken. The organs 

 are composed of four classes of tissues : Epithelial, which 

 covers all free surfaces; connective, forming the bones, 

 binding together the muscular fibers and elements of the 



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