AN AMERICAN 



TEXT-BOOK OF PHYSIOLOGY. 



I. INTRODUCTION. 



THE term "physiology" is, in an etymological sense, synonymous with 

 " natural philosophy," and occasionally the word is used with this significance 

 even at the present day. 1 By common usage, however, the term is restricted 

 to the living side of nature, and is meant to include the sum of our know- 

 ledge concerning the properties of living matter. The active substance of 

 which living things are composed is supposed to be closely similar in all cases, 

 and is commonly designated as protoplasm (Tr/xwroc, first, and xMafia, any- 

 thing formed). It is usually stated that this word was first introduced into 

 jiological literature by the botanist Von Mohl to designate the granular semi- 

 liquid contents of the plant-cell. It seems, however, that priority in the use 

 of the word belongs to the physiologist Purkinje (1840), who employed it to 

 describe the material from which the young animal embryo is constructed. 2 

 In recent years the term has been applied indifferently to the soft material 

 constituting the substance of either animal or plant-cells. The word must not 

 be misunderstood to mean a substance of a definite chemical nature or of an 

 invariable morphological structure ; it is applied to any part of a cell which 

 shows the properties of life, and is therefore only a convenient abbreviation 

 for the phrase " mass of living matter." 



Living things fall into two great groups, animals and plants, and corre- 

 sponding to this there is a natural separation of physiology into two sciences, one 

 dealing with the phenomena of animal life, the other with plant life. In what 

 follows in this introductory section the former of these two divisions is chiefly 

 considered, for although the most fundamental laws of physiology are, without 

 doubt, equally applicable to animal and vegetable protoplasm, nevertheless the 

 structure as well as the properties of the two forms -of matter are in some 

 respects noticeably different, particularly in the higher types of organisms in 

 each group. The most striking contrast, perhaps, is found in the fact that 

 plants exhibit a lesser degree of specialization in form and function and 



1 See Mineral Physiology and Physiography, T. Sterry Hunt, 1886. 



2 O. Hertwig : Die ZeUe und die Gewebe, 1893. 



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