18 INTRODUCTION. 



if the heart is motionless, we say that the animal is dead. This 

 beating of the heart, therefore, is a vital phenomenon that is, 

 a manifestation of life. We speak also of this beating of the 

 heart as its function ; hence the first definition of physiology, 

 that it is the science which treats of functions, and the second 

 definition, that it is the science which treats of vital phenomena, 

 amount to the same thing. 



Definition of "Organ." An organ has already been defined as 

 a structure which performs a function or functions. In speaking 

 of the organs of an animal reference is usually had to such struct- 

 ures as the heart, the lungs, and the stomach, inasmuch as their size 

 and the important work they perform force them upon our attention. 

 These are indeed organs, for they perform functions ; thus the 

 function of the heart is to receive blood in one portion and to 

 propel it from another portion, that of the lungs is to aerate the 

 blood, and that of the stomach is to digest certain kinds of food ; 

 but the term organ, as used in physiology, has a much broader 

 signification. A muscle, a nerve, and a blood-vessel are as truly 

 organs as are the greater ones above spoken of, for each has its 

 own function. Thus the function of a muscle is to contract, that 

 of a nerve is to transfer nervous impulses, and that of a blood- 

 vessel is to convey blood. At first sight it might seem that these 

 functions were unimportant, and that the structures which per- 

 formed them were hardly worthy of so dignified a name as organs ; 

 but a moment's reflection will show that without the contraction 

 of muscles, the transference of nervous impulses, or the carrying 

 of blood the life of an animal would as certainly cease as if it 

 was deprived of its heart, of its lungs, or of its 'stomach. 



Inasmuch as minerals, on the one hand, possess no organs, 

 they perform no work that is, they have no functions ; therefore 

 we do not speak of the physiology of a mineral. Plants and ani- 

 mals, on the other hand, possess organs, each of which performs its 

 special function ; and it is with them, as has been said, that 

 physiology has to do. As we find organs in the animal, so do we 

 find them in the plant ; not the same organs, it is true, but struct- 

 ures which are as truly organs, for they respond to the same test. 

 The roots of a plant absorb moisture and nourishment from the 

 soil, this being their function ; the green leaves take up from 

 the air carbonic acid, with which and with water they form starch 

 that is utilized by the plant, while oxygen is set free, this being 

 the function of the leaves ; the anthers and the ovaries of flowers 

 are concerned in reproducing plants by forming new ones, this 

 being their function. Thus we might continue to show that as in 

 animals, so in plants, the different organs have their respective 

 functions. 



Definition of "Organic" and "Inorganic." We can now under- 

 stand the meaning of two very important terms organic and 



