18 GENERAL INTRODUCTION 



His system of medicine, from which the physiology of the 

 time is inseparable, held an almost indisputable sway for nearly 

 thirteen centuries. Harvey (1578-1657 A.D.), whose name 

 stands foremost among those of his time, discovered the cir- 

 culation of the blood. His greatest accomplishment was the 

 establishment of the experimental method in physiology upon 

 a firm basis. With him originated the conception " omne vivum 

 ex ovo." Holler (1708-1777 A.D.) was the first to recognize 

 the necessity of bringing together the mass of physiological 

 facts and- theories that had arisen during the sixteenth and 

 seventeenth centuries into an independent science. This he 

 did in his Elementa Physiologies Corporis Humani. Johannes 

 Muller (1801-1858 A.D.) was perhaps the greatest physiologist 

 of all times. He impressed upon his science the general form 

 or aspect that it wears today. 



The aim of physiology is the investigation of life. The term 

 life is, however, not readily definable. In general, any given 

 piece of matter is said to be alive when it manifests the funda- 

 mental properties of living things. These properties may be 

 defined as follows: 



1 . Irritability is that property of protoplasm which enables it 

 to undergo characteristic physical and chemical changes when 

 acted upon by certain influences called stimuli. Usually there 

 is a liberation of energy in the response out of all proportion 

 to the energy applied in the stimulus. 



2. Conductivity is that property of protoplasm by virtue 

 of which a condition of activity aroused in one portion of the 

 substance may be transmitted to any other portion. 



3. Contractility is that property of protoplasm which enables 

 it to change its form when irritated by stimuli. 



4. Nutrition is that property of protoplasm which enables it 

 to convert dead food material into its own living substance. 



5. Reproduction is that property of protoplasm which enables 

 it to separate into a number of parts, each of which may develop 

 into the parent form. None of the fundamental properties 

 serve absolutely to distinguish living from dead matter, since 

 all are simulated more or less completely by phenomena in 

 the non-living world. 



Life is always associated with a peculiar form of matter 

 called protoplasm, and is never found elsewhere. 



