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 liviug 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 fundamentally alike in 

 structure in all cases, and is commonly designated as protoplasm {-oioroz, first, 

 and nXdofxa, anything formed). It is usually stated that this word was first 

 introduced into biological literature by the botanist Von Mold 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 indif- 

 ferently to the soft material constituting the substance of either animal or 

 plant-cells. The word must not be understood to mean a substance of a 

 definite chemical nature or of an invariable morphological structure ; it is 

 applied to any part of a cell that 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 plant's, 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 fad that 

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



1 See Mineral Physiology ami Physiography, 'I'. Sterry Hunt, L886. 



2 O. Hertwig: Die '/Ale and die (,'ewehe, lS<t.">. 



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