44 HUMAN PHYSIOLOGY 



single cells are doubtless of this kind, save in those 

 cases in which water enters or leaves. This exceptional 

 possibility we shall not be obliged to consider in the 

 present treatment of the subject. 



Many of the facts of life which we assume for free- 

 living cells can hardly be demonstrated but are rather 

 inferred from what we know of higher organisms. The 



FIG. 4. The purpose of this diagram is to assist the student in 

 gaining notions of scale. The large circle stands for the cross-section 

 of a fine hair. Its diameter is supposed to be 3^00 inch. Within it 

 (bl) is a red blood corpuscle, /^200 i nc ^ across. The budding yeast-cell 

 (y) is of a similar order of magnitude. Bacteria (bact.) are a good deal 

 smaller. 



production of heat, for example, is something we can 

 scarcely hope to measure when only one cell is concerned, 

 but we cannot doubt that it is going on because we know 

 that it is evident when sufficiently large numbers of 

 active cells are massed. The phenomena we can most 

 easily make out in a study of solitary cells are the taking 

 of food, motion, and reproduction. A word may be 

 said about the last-named manifestation of life. 



Cell Reproduction. A cell which is well nourished and 

 otherwise in a favorable condition soon attains to a size 

 which marks the limit for its growth. Instead of in- 

 creasing further in bulk it cleaves into two parts which 

 are complete and living cells. The cells of the new 



