BRANCHES OF PHYSIOLOGY. 19 



inorganic. These terms are used in two senses : first, as to struct- 

 ure, and, second, as to product. When we say that a plant or an 

 animal is organic, we mean that it is made up of organs that is, 

 of structures which perform functions. The plant or the animal 

 may be simple or may be complex, but, however simple or 

 however complex, its parts do something, that something being 

 the function of the part which acts. We say, therefore, that 

 the plant or animal is organic, meaning that it is composed of 

 organs organic, then, as to structure. The rock has no organs, 

 therefore it is non-organic, or is inorganic. These terms are used 

 also in another sense. Thus we speak of honey as organic. Mani- 

 festly, we do not mean organic as to structure, for honey has no 

 organs, that is, no parts which perform functions, but it is the 

 product of the bee, which is an organic structure ; hence honey 

 is an organic product. The nectary of a flower is organic as to 

 structure, and the nectar which it produces is also organic, 

 inasmuch as it is the product of the nectary. 



But organs do not act each for itself: they are, as a rule, 

 associated in the performance of a common function, and thus 

 associated form a system. Thus the group of organs which are 

 concerned in digestion forms the digestive system ; those which 

 together accomplish the circulation of the blood, the circulatory 

 system. An attempt has been made to distinguish an apparatus 

 from a system; the former being defined as a group of organs 

 concerned in the performance of a common function, no matter 

 how dissimilar their structure, while organs similar in structure 

 irrespective of their function would be regarded as a system. 

 Similarity of function, under this definition, would characterize 

 an apparatus, and similarity of structure a system. The organs 

 whose functions are to digest food would be regarded as an appa- 

 ratus, constituting the digestive apparatus ; the bones, on the other 

 hand, would form the osseous system. Practically, however, such 

 a differentiation is of no use, and the two terms apparatus and 

 system may therefore be used interchangeably. , 



Branches of Physiology. From these elementary con- 

 siderations it is evident that physiology has to do with living 

 plants and animals only that is, with organic structures and inci- 

 dentally with their products. That branch of the science which 

 treats of the functions of plants is denominated Vegetable Physi- 

 ology, and that which deals with the functions of animals is called 

 Animal Physiology. 



Vegetable Physiology. We are concerned but indirectly with 

 vegetable physiology, or so far only as its study helps us to under- 

 stand some of the more obscure processes in animals. Some of these 

 processes, being simpler in plants, are more easily studied in them, 

 and what is there learned is of great assistance in understanding 

 analogous processes in man. Thus a knowledge of fertilization as 



