52 THE CELL 



of these unstable combinations, molecules of greater stability and 

 less complexity are formed. This results in the transformation of 

 potential into kinetic energy, and the expenditure of this energy is 

 expressed in function. 



3. Irritability is that property which enables a cell to respond to 

 external stimuli. Cells vary in respect to their irritability, the most 

 markedly irritable cells in higher animals being those of the neuro- 

 muscular mechanism. Stimulation may be mechanical, electrical, 

 thermal, chemical, etc. The response of the cell to certain forms of 

 chemical stimulation is known as cheniotaxis. Some substances 

 attract cells (positive cheniotaxis); others repel cells (negative 

 chemotaxis). Stimuli other than chemical possess similar properties, 

 as indicated by the terms thermotaxis, galvanotaxis, etc. Some cells 

 are so specialized as to react only to certain kinds of stimulation, e.g., 

 the retinal cells only to light stimuli. 



^^^. 



m 







WMi^ 







Fig. 6. — Amoeboid ^Movement. Successive changes in shape and position of 



fresh-water amceba. 



4. Motion. — This is dependent wholly upon the protoplasm of the 

 cell, and is exhibited in several somewhat different forms. 



(a) Amxhoid Movement. This consists in the pushing outward 

 by the cell of processes (pseudopodia) . These may be retracted or 

 may draw the cell after them. In this way the cell may change both 

 its shape and position (Fig. 6). While possessed by many animal 

 cells, this property of amoeboid movement is most marked in such 

 cells as the leucocytes or white blood cells. By their amoeboid move- 

 ments these cells are able to pass out of the blood vessels and as so- 

 called ''wandering cells" to circulate in the tissues. By means of 

 their pseudopodia they can also surround tiny objects^ — granules, 

 bacteria, etc., and sometimes digest or destroy them. Such cells are 

 known as phagocytes and the phenomenon as phagocytosis. They 

 play an extremely important role in the body economy both in 

 health and disease. 



{h) Protoplasmic Movement. This occurs wholly within the lim- 



