FUNCTION AND STRUCTURE 65 



physiology and the physiologists little to anatomy. But 

 we have seen how inseparably linked are structure and 

 function. The structure of an animal is as it is because 

 of the work it has to do, and the functions of an animal 

 are performed as they are performed because of the special 

 structural condition of the organs which perform them. 

 The study of the anatomy and the study of the physiology 

 of animals can not be separated. To understand aright 

 the structure of an animal it is necessary to know to 

 what use the structure is put ; to understand aright the 

 processes of an animal it is necessary to know the struc- 

 ture on which the performance of the processes depends. 



40. The animal body a machine. The body of an animal 

 may be well compared with some machine like a locomotive 

 engine. Indeed, the animal body is a machine. It is a 

 machine composed of many parts, each part doing some 

 particular kind of work for which a particular kind of 

 structure fits it ; and all the parts are dependent on each 

 other and work together for the accomplishment of the 

 total business of the machine. The locomotive must be 

 provided with fuel, such as coal or wood or other readily 

 combustible substance, the consumption of which furnishes 

 the force or energy of the machine. The animal body 

 must be provided with fuel, which is called food, which 

 furnishes similarly the energy of the animal. Oxygen must 

 be provided for the combustion of the fuel in the locomo- 

 tive and the food in the body. The locomotive is com- 

 posed of special parts : the firebox for the reception and 

 combustion of fuel ; the steam pipes for the carriage of 

 steam ; the wheels for locomotion ; the smoke stack for 

 throwing off of waste. The animal body is similarly com- 

 posed of parts : the alimentary canal for the reception and 

 assimilation of food ; the excretory organs for the throwing 

 off of waste matter ; the arteries and veins for the carriage 

 of the oxygen and food-holding blood ; the legs or wings 

 for locomotion. 



