26 NORMAL HISTOLOGY. 



least commonly present of all, and there are certain 

 cells which consist of a cell-body alone. 



Regarding cells now from a physiological point of 

 view, we find the expression of their vitality in four 

 distinct ways : they nourish themselves; they grow ; 

 they perform certain functions for their own good and 

 for the benefit of the organism of which they form a 

 part ; and, finally, under certain circumstances, they 

 are capable of reproducing their like. Or, in more 

 concise language, we say the cell expresses its 

 vitality in nutrition, growth, function, and reproduc- 

 tion. Not all of these expressions of vitality, how- 

 ever, can be subjected to direct microscopical 

 observation. Nutrition, being as we believe essen- 

 tially a chemical process, cannot, with our present 

 facilities, become to any considerable extent the 

 object of direct microscopical study. The growth of 

 cells, too, is for the most part so gradual, that we 

 cannot directly follow it, but are obliged to study it 

 in different phases of its progress. 



The functional activity of cells can be indirectly 

 subjected to microscopical investigation when it is 

 associated with demonstrable changes in the mor- 

 phological characters of the cell, or directly observed 

 when it expresses itself in motion ; thus, we can 

 readily detect the difference between a condition of 

 functional activity and rest in the peptic cells of 

 the stomach, and the observation of ciliary and 

 amoeboid movements in certain cells is among the 

 most fascinating of histological studies. 



