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. 
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