24 VETERINARY PATllULOGV. 



dependent upon frequent and appropriate exercise (liljeration of 

 kinetic energy ) . 



Growth within the normal cell is dependent upon inherited 

 tendencies and a sufficient suppljf of nutrition. Other functions, 

 as motion, are apparentl3f entirely governed by environmental 

 stimuli plus the required nutrition. The growth of cells con- 

 tinues until they, and the part they compose, become of such 

 a size that tlie economic relation of surface and mass becomes 

 disproportionate. The disproportionate relation of surface to 

 mass is corrected by rapid cell division or cell dissociation. In 

 either case tlie total cell surface is increased. According to 

 Harris. "I'hysiologic inertia" is of considerable importance in 

 growtli of cells. When a cell is stimulated to action, the action 

 does not cease immediately when the stimulus is removed or 

 suspended. Thus when a cell starts to grow, it tends to grow 

 continualh' 1)ccause of the "physiologic inertia." Abnormal varia- 

 tion in cell growth is characteristic of hypertroph}'. 



Reproduction. — Cell reproduction is the process bv which 

 the number of cells is increased. The ultimate outcome of cell 

 reproductii in and cell growth, is to increase the mass or volume. 

 Cell reproduction is not distinct and separable from cell growth, 

 in fact growtli always precedes di\'ision. Reproduction is one 

 means of regulating the relation of surface to mass. Two tvpes 

 of normal cell reproduction have been described bv cvtologists. 

 These methods are amitosis, (direct cell division), and mitosis, 

 (indirect cell division.) 



1. I'lRi-x'T CELL DIVISION, AiiiTOSis, is simple cell division in 

 which the nucleus and cell body divide without any previous 

 nuclear changes, litis type of cell reproduction or division is 

 ncirmal in some lower forms of life and possiblv in some embrv- 

 onic tissues of liigher animals. However, it is not very common 

 in normal adult tissues of higher animals. The polynuclear 

 leucocytes occasionally reproduce bv amitosis; endothelial cells 

 are also tjiought to reproduce in tlie same way. It is possible 

 that cells (.f any tissue may multiplv bv amitosis. Cells repro- 

 duced ]iy amitr.tic dix'ision are considered abnormal by inost 

 investig.'itors. Direct cell division is especially evidenced in 

 rapidly growing tumors, chronic inflanimatorv areas, leukemic 

 tissue and many other pathologic conditions." The process is 

 briefly as frillows ; There are one or more depressions in the 

 nucleus whicli graduallv extend until the nucleus is divided into 

 two or more parts, (this is tlie origin of polynuclear cells; possi- 

 bly it may also account for the giant cells)", After, the nucleus 



