V1 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 clue. 

 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 



