20 MANUAL OF PHYSIOLOGY. 



generally, impossible to say exactly how they are brought about, 

 and we are therefore obliged, for the present at least, to be satis- 

 fied with the mere recognition and description of the phenomena. 

 Since the human organism is the special study of students of 

 medicine, the contents of this volume should properly be restricted 

 to the physiology of man. But human physiology cannot be 

 studied alone : because in man we cannot watch sufficiently closely, 

 or question fully, by experiment, the phenomena of life. Further, 

 no sharp line of separation can be drawn between the actions of the 

 various organs of man and those of the lower animals. The consid- 

 eration of the physiology of those animals which are akin to man 

 must therefore go hand in hand with the study of the physiology 

 of man himself. Much light has been thrown on the actions of 

 the most complex textures of the highest animals, by the obser- 

 vation of the activities of the lowest organisms, where the mani- 

 festations of life may be carefully watched with the microscope in 

 the living animal under perfectly normal conditions. 



GENERAL CHAEACTERS OF ORGANISMS. 



The term organism, which is commonly used as having the 

 same meaning as living being, owes its derivation to the complex- 

 ity of structure common among the higher forms of life, which 

 are made up of several distinct organs. This organic construction 

 no longer holds good as a distinguishing mark between living 

 beings and inanimate matter, because we are acquainted with a 

 vast number of living organisms, both plants and animals, which 

 are not made up of organs, but are composed of a minute piece 

 of a soft jelly-like material, which is simply granular throughout, 

 and devoid of structural differentiation during the life of the 

 creature. 



We may classify the general characters of living beings as fol- 

 lows: 



1. Structural and physical properties. 



Chemical composition. 



3. Activities during life (vital phenomena). 



1. Structural Characters of Organisms. The minute 

 structure of living beings as shown, by the microscope no doubt 



