26 HUMAN PHYSIOLOGY. 



and chemic composition in the different tissues of the adult body, they are 

 nevertheless, descendants from typical cells, known as embryonic or undiffer 

 entiated cells, the first offspring of the fertilized ovum. Ascending the line 

 of embryonic development, it will be found that every organized body origi- 

 nates 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 internal 

 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 cyto- 

 plasm or bioplasm, in which is embedded a smaller spheric body, the nucleus. 

 Within the nucleus there is frequently a still smaller body the nucleolus. 

 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 

 -gfca of an inch, the diameter of a red blood-corpuscle, to ^ of an inch, 

 the diameter of the large cells in the gray matter of the spinal cord. (See 

 Fig. 2). 



The cell cytoplasm consists of a soft, semifluid, gelatinous material, vary- 

 ing somewhat in appearance in different tissues. Though frequently homo- 

 geneous, it often exhibits a finely granular appearance under medium powers 

 of the microscope. Young cells consist almost entirely of clear cytoplasm, 

 mature cells contain, according to the tissue in which they are found, material 

 of an entirely different character e. g., small globules of fat, granules of 

 glycogen, mucigen, pigments, digestive ferments, etc. Under high powers 

 of the microscope the cytoplasm is found to be pervaded by a network of 

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

 and more fluent substance, the hyaloplasm. The relative amount of these 

 two constituents varies in different cells, the proportion of hyaloplasm being 

 usually greater in young cells. The arrangement of the fibers forming the 

 spongioplasm also varies, the fibers having sometimes a radial direction, in 

 others a concentric disposition, but most frequently being distributed evenly 

 in all directions. In many cells the outer portion of the cell cytoplasm under- 

 goes chemic changes and is transformed into a thin, transparent, homogenous 

 membrane the cell membrane which completely incloses the cell substance. 

 The cell membrane is permeable to water and watery solutions of various inor- 

 ganic and organic substances. It is, however, not an essential part of the cell. 



