24 PRINCIPLES OF PHYSIOLOGY 



and water, and energy is liberated as heat, 

 which may be transformed into the motion 

 of a steam engine and caused to do work. 

 Living matter is thus continually undergoing 

 a series of chemical changes of composition 

 and decomposition, as a result of which there 

 is an incessant renovation of its particles. 

 It would appear that chemical changes are a 

 necessary condition of the action of living 

 matter ; part of the living matter dies, is 

 decomposed, or rather, its decomposition is 

 coincident with its death, and the dead matter 

 is thrown out of the body. New matter is 

 then added from the outer world. There is 

 thus a perpetual exchange between the dead 

 and the living worlds, which may be termed a 

 circulation of matter between the dead and the 

 living. Portions of the earth's surface now dead 

 were once part of the bodies of living beings, 

 and may again enter into the living state. 



10. Form and Mode of Growth. As already 

 pointed out, living matter is jelly-like in its 

 consistence. It often changes its form, as 

 may be seen in an amoeba or in the colourless 

 cells in the blood. At the beginning of 



