2 NORMAL HISTOLOGY. 



using this misleading term in its accepted but not literal sense. The sub- 

 stance of the entire cell, including that of the cell-body and of the nucleus, 

 is the protoplasm, the cell being often defined as "a minute nucleated particle 

 of protoplasm. ' ' 



THE STRUCTURE OF THE CELL. 



The Cytoplasm. The translucent, viscid substance forming the cell- 

 body, the cytoplasm, is complex in both its chemical and structural com- 

 position. Chemically, cytoplasm consists of a heterogeneous mixture of 

 water, salts and organic compounds. The latter exist chiefly as proteins, 

 which are complex combinations of carbon, hydrogen, nitrogen and oxy- 



Exoplasm 



Cytoplasm 



Karyosome 



Linin thread 

 Chromatin 



Spongioplastn 

 Hyaloplasm 



Metaplastic inclusions 



Endoplasm 



Nuclear membrane 

 Nucleolus 



j Centrosome surrounded by 



centrosphere 



Cytoplasm 



Cell-wall 



FIG. 2. Diagram of cell-structure. In the upper part of figure the granular condition of the cytoplasm 

 is represented; in the lower and left part, the reticular condition. 



gen, with a small percentage of phosphorus and of sulphur. The inorganic 

 substances include sodium, potassium, calcium, magnesium, chlorine and 

 usually iron. 



The cytoplasm by no means always presents the same structural appear- 

 ance, since its constituents are subject to changes in their condition and 

 arrangement which produce corresponding morphological variations. Thus, 

 the cytoplasm may be devoid of recognizable definite structure and appear 

 homogeneous; at other times it may present aggregations of minute spherical 

 particles and then be described as granular, or where the minute spheres are 

 larger and consist of fluid substances embedded within a surrounding denser 

 material, as alveolar; or, again, and very frequently, the cytoplasm contains 

 a meshwork of threads or fibrils, more or less conspicuous, which arrange- 

 ment gives rise to the reticular condition. It must be recognized, therefore, 

 that the structure of cytoplasm is not to be regarded as immutable, but 

 on the contrary, as capable of undergoing changes which render it probable 

 that a cell may appear during one stage of its existence as granular and at 

 another period as reticular. 



Whatever be the particular phase of structure exhibited by the cell, 

 histologists are agreed that the cytoplasm consists of two substances an 

 active and a passive; while both must be regarded as living, contractility is 

 probably due particularly to the former. 



