PHYSIOLOGY OF THE CELL. 29 



PHYSIOLOGY OF THE CELL. 



A microscopic analysis of the tissues shows that they can be 

 resolved into simpler elements, termed cells, which may, therefore, be 

 regarded as the primary units of structure. Though cells vary con- 

 siderably in shape, size, and chemic composition in the different 

 tissues of the adult body, they are, nevertheless, descendants from 

 typical cells, known as embryonic or undifferentiated cells, examples 

 of which are the leukocytes of the blood and lymph and the first 

 offspring of the fertilized ovum. Ascending the line of embryonic 

 development, it will be found that every organized body originates 

 in a single cell the ovum. As the cell is the elementary unit of all 

 tissues, the function of each tissue must be referred to the function 

 of the cell. Hence the cell may be defined as the primary anatomic 

 and physiologic unit of the organic world, to which every exhibition 

 of life, whether normal or abnormal, is to be referred. 



Structure of Cells. Though cells vary in shape and size and in- 

 trenal structure in different portions of the body, a typical cell may 

 be said to consist mainly of a gelatinous substance forming the body 

 of the cell, termed protoplasm or bioplasm, in which is embedded a 

 smaller spheric body, the nucleus. The shape of the adult cell 

 varies according to the tissue in which it is found ; when young and 

 free to move in a fluid medium, the cell assumes a spheric form, 

 but when subjected to pressure, may become cylindric, fusiform, 

 polygonal, or stellate. Cells vary in size within wide limits, ranging 

 from^J^ of an inch, the diameter of a red blood-corpuscle, to ^J 7 of 

 an inch, the diameter of the large cells in the gray matter of the 

 spinal cord. (See Fig. 2.) 



The cell protoplasm consists of a soft, semifluid, gelatinous material, 

 varying somewhat in appearance in different tissues. Though fre- 

 quently homogeneous, it often exhibits a finely granular appearance 

 under medium powers of the microscope. Young cells consist almost 

 entirely of clear protoplasm, mature cells contain, according to the 

 tissue in which they are found, material of an entirely different char- 

 acter e. g., small globules of fat, granules of glycogen, mucigen, 

 pigments, digestive ferments, etc. Under high powers of the micro- 

 scope the cell protoplasm is found to be pervaded by a network of 

 fibers, termed spongioplasm, in the meshes of which is contained a 



