CHAPTER II. 



ORGANS, TISSUES, AND CELLS. — EPITHELIAL TISSUES: STRATI- 

 FIED, TRANSITIONAL, SIMPLE. 



Organs, tissues, and cells. — In speaking of the different parts 

 of the body, we usually call each part an organ, and we may say 

 that the human body is made up of organs, each organ being 

 adapted to the performance of some special work or function. 

 Thus the lungs are organs specially adapted for performing 

 the function of respiration, the bones are organs adapted for 

 support and locomotion, the kidneys for secreting urine, etc. 



Every part or organ, when examined microscopically, is found 

 to consist of certain textures or tissues. When the body is 

 thus analyzed by the aid of the microscope, we find that the 

 number of distinct tissues is comparatively small, and some of 

 these again, although at first sight apparently distinct, yet have 

 so much in common in their structure and origin one with 

 another, that the number becomes still further reduced, until 

 we can only distinguish four distinct tissues, viz. : — 



The epithelial tissues. The muscular tissues. 



The connective tissues. The nervous tissues. 



Particles met with in the fluids of the body, such as the little bodies or cor- 

 puscles in the blood and lymph, are also reckoned among these elementary 

 tissues. 



Some organs are formed of a combination of several of the 

 above tissues ; others contain only one or two. Thus the 

 muscles are made up almost entirely of muscular tissue with 

 only a small intermixture of connective tissue, blood-vessels, 

 and nerves ; whilst the ligaments or sinews are composed 

 wholly of a variety of connective tissue. 



On the other hand, there are certain organs or parts of the 



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