VI FORM AND FUNCTION 141 
Swan shows convolutions of the trachea very similar 
to those of the Crane. Whatever other purpose it may 
serve, the long coiled windpipe ensures the thorough 
warming of the air before it reaches the lungs. 
Muscles and Tendons. 
To muscles all movement in the body is due. 
When acted on by the motor nerves they contract 
and become shorter, with the result that the bone or 
other organ connected with them is moved. The 
nerve, in reality, gives a series of small shocks which 
owing to the elasticity of the muscle, result in one 
movement. The diminution in length of the con- 
tracting muscle is balanced by an increase in breadth 
and thickness. Great as its force is, it is not a perfect 
machine. Like a steam-engine it only converts a 
fraction of its total energy into work, the rest taking 
the form of heat. In a steam-engine the work done 
is rarely more than one-tenth of the total energy. In 
a muscle, as far as we know at present, it varies from 
one fourth to one twenty-fourth. 
There are two kinds of muscles: (1) striated or 
striped ; (2) unstriated or smooth. All muscles which 
we move voluntarily are striated, and it is these which 
move most quickly. The unstriated muscles, on the 
other hand, which aid in carrying on the processes of 
life in the body, move slowly and are subject to the 
sympathetic system of nerves which are not under 
the control of the will. The muscle of the Iris in man is 
altogether exceptional ; it is unstriated ; its action is in 
some cases voluntary, in others involuntary, according 
