CHAPTER III 



CELLS, TISSUES, ORGANS, AND SYSTEMS. — EPITHELIAL TISSUE: 

 SIMPLE, TRANSITIONAL, STRATIFIED 



From the standpoint of the chemist the body is composed of 

 elements. (See page 3.) From the standpoint of the anatomist 

 the body is composed of cells, and they are regarded as the struc- 

 tural units out of which either directly or indirectly it is built. 

 If the substance of any part of the body, i.e. skin, muscle, or blood, 

 is examined with the imaided eye, it appears homogeneous, but 

 if examined with the microscope it is found to be composed of 

 an innumerable number of these minute cells. , It is helpful to re- 

 call that low down in the scale of life we find animals so simple 

 that they are described as consisting of just one cell. As we 

 ascend in the scale of life, we find animals that consist of a greater 

 number of cells, until the human body may be properly described 

 as an enormous aggregate of cells. 



All the varied activities of the body are the result of the activity 

 of the cells which compose it, and it is very desirable that we early 

 acquire some definite conception of these tiny elementary bodies. 



CELLS 



A cell ^ is a minute portion of li\'ing substance or protoplasm 

 which is sometimes enclosed in a cell membrane or cell wall. 

 Within the protoplasm lies a body of definite rounded form, called 

 the nucleus, and this in turn often contains one or more smaller 

 bodies or nucleoli. As the substance of the entire cell is proto- 

 plasm, that portion which surrounds the nucleus is given the 

 name cytoplasm, and the substance of the nucleus is named 

 karyoplasm. 



1 The word cell is from the Lutin cella — a cavity — and was first used by bot- 

 anists to describe plant cells, like those of cork and elder pith, which have cavities 

 in their substance. It is now known that most animal cells, and many plant cells, 

 do not have cavities, so that the name is not especially appropriate, but it is too 

 firmly fixed in our language to be abandoned. 



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