10 VETERINARY PHYSIOLOGY 



undergoing internal changes. It might be compared to a 

 whirlpool constantly dragging things into its vortex, and 

 constantly throwing them out more or less changed, but 

 itself continuing apparently unchanged throughout. Hoppe- 

 Seyler expresses this by saying : " The life of all organisms 

 depends upon, or, one can almost say, is identical with, a 

 chain of chemical changes." Foster puts the same idea in 

 more fanciful language : " We may speak of protoplasm as a 

 complex substance, but we must strive to realise that what 

 we mean by that is a complex whirl, an intricate dance, of 

 which what we call chemical composition, histological 

 structure, and gross configuration are, so to speak, the 

 figures." 



Death. — While the continuance of these chemical changes 

 in protoplasm is life, their stoppage is death. For the con- 

 tinuance of life the building-up changes must be in excess 

 of, or equal to, the breaking-down — the evolution of energy 

 must be sufficient for growth or maintenance. It is only 

 the surplus over this which is available for external work. 

 In the young the surplus energy is largely used for growth 

 and develoj)ment ; in adult life for work. When failure in 

 the supply or in the utilisation of the energy-yielding 

 material occurs, the protoplasm dwindles and disintegrates. 

 Death is sudden when the chemical changes are abruptly 

 stopped, slow when the anabolic changes are interfered with. 



The series of changes which occur between the infliction 

 of an incurable injury and complete disintegration of the 

 protoplasm constitute the processes of Necrobiosis, and their 

 study is of importance in pathology. 



Stimuli. — The rate of the chemical processes in protoplasm 

 may be quickened or slowed by changes in the surroundings, 

 and such changes are called stimuli. If the stimulus increases 

 the rate of change, it is said to excite ; if it diminishes the 

 rate of change, it is said to depress. Thus the activity of 

 the changes in yeast may be accelerated by a slight increase 

 of the temperature of the surrounding medium, or it may be 

 depressed by the addition of such a substance as chloroform. 



