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 
Fic. 7. Frat EpirHetium CELLs Fic. 8. INvoLtuNTARY MUSCLE- 
FROM THE MoutTH. 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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