HUMAN PHYSIOLOGY. 



INTRODUCTION, 



Definitions. Physiology is the science which treats of func- 

 tions. By the term function is meant the characteristic work 

 performed by an organ. An organ may be defined as a structure 

 which performs a function or functions, for the special or char- 

 acteristic work of an organ may not be limited to a single 

 function : thus the pancreas secretes not only pancreatic juice, 

 which is its external secretion, but also another product, which is 

 its internal secretion (p. 239). Lifeless things perform no functions, 

 hence physiology has no dealings with inanimate things. Rocks, 

 stones, and other members of the mineral kingdom at no time 

 possess life ; consequently they perform no functions, and with 

 them physiology has no concern : we cannot speak of the physi- 

 ology of minerals. Plants and animals are sometimes living and 

 sometimes dead : when living they perform functions, when dead 

 they perform no functions ; in the latter condition they are like 

 the rocks so far as function is concerned, and with them physiology 

 has nothing whatever to do. It is only when they are living that 

 they perform functions, and it is then and only then that with 

 them physiology concerns itself. 



Another definition which might be given of physiology is, 

 that it is the science which treats of vital phenomena. A brief 

 consideration of this definition will bring us to the same conclu- 

 sion as did that of the preceding definition. Of life in its essence 

 we know nothing. Metaphysicians have endeavored to explain 

 life, and some have even ventured to point out its seat, but the 

 fact remains that we are utterly ignorant of its nature. We only 

 know that it exists by certain manifestations which it presents. 

 When we see a growing plant or a moving animal, we say of each 

 that it is alive. In the higher forms of animals and plants it is 

 easy, under ordinary circumstances, to determine whether they are 

 living or not ; but in the lower forms this determination is some- 

 times a most difficult task. The evidences upon which reliance 

 is placed to determine the presence or the absence of life are 

 spoken of as vital phenomena. Thus, if in examining an animal 

 we find that its heart beats, we say that the animal is alive ; but 



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