120 General Biology 



The various organs of the body are responsible for the particular 

 size, shape, and function of the animal possessing them. 



There are two ways of looking at an organ : 



(a) Morphologically, or according to its structure or anatomy. 



(b) Physiologically, according to the function such organ 

 may perform. 



If organs of different animals are physiologically equivalent, that 

 is, if they function similarly, they are known as analogous organs. 



If the organs of different animals are morphologically equivalent, 

 that is, if they have developed in a similar manner in relation to the 

 other structures immediately surrounding them, they are called homolo- 

 gous organs. 



There are three possibilities in comparing animals : 



(a) The organs may at the same time be homologous and 

 analogous. 



(b) They may be homologous but not analogous, as for exam- 

 ple the swim-bladder of fishes and lungs of mammals. 



(c) They may be analogous but not homologous, as for exam- 

 ple the gills of fishes and the lungs of mammals. 



The functions of organs are said to be : 



(a) Vegetative (as in plants), when they have to do principally 

 with growth. 



(b) Animal, referring to those functions which are absent in 

 plants or but very slightly developed. In the animal kingdom they are 

 considerably increased or are totally separate and distinct from any- 

 thing the vegetable world may possess. 



The vegetative functions are equally complete in both man and the 

 lower animals although they may develop quite differently in the two 

 groups. 



Animal functions are those of motion and sensation. The work of 

 the various specialized sense organs, such as the eye and ear, come under 

 this grouping, while the work of those organs which pertain to nutrition 

 and reproduction, which both plants and animals possess equally well, 

 are vegetative. 



Living matter has been shown to have four distinguishing charac- 

 teristics : 



(a) Irritability, 



(b) Growth by intussusception, 



(c) Reproduction, 



(d) Nutrition. 



When nutrition is discussed biologically, it must be thought of in 

 its widest sense as including not only the taking in of food and drink, 



