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 elim- 

 ination of the nitrogenous waste. Several organs com- 

 bined for a common purpose constitute a system. The 

 heart, with the various vessels for conveying the blood, 

 forms the circulatory system. The following eight 

 systems are found in all Mammalia : Osseous or bony, 



n 





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. 



muscular, digestive, respiratory, vascular, excretory, re- 

 productive, and nervous. 



The relative locations of the various systems are 

 represented 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 

 most cases one can scarcely distinguish them by micro- 

 scopic examination. The organs are composed of four 



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