WALKING 329 



And, as the swinging leg is a true pendulum — the time of 

 vibration of which depends, other things being alike, 

 upon its length (short pendulums vibrating more quickly 

 than long ones), — it follows that, on the average, the 

 natural step of short-legged people is quicker than that 

 of long-legged ones. 



In running, there is a period when both legs are oflF the 

 ground. The legs are advanced by muscular contraction, 

 and the lever action of each foot is swift and violent. 

 Indeed, the action of each leg resembles, in violent 

 running, that which, when both legs act together, consti- 

 tutes a jicmj), the sudden extension of the legs adding 

 to the impetus, which, in slow walking, is given only by 

 the feet. 



14. The Mechanism of the Larynx.— Perhaps the 

 most singular motor apparatus in the body is the larynx, 

 by the agency of which voice is produced. 



The essential conditions of the production of the human 

 voice are : — 



(a) The existence of the so-called vocal cords. 



(b) The parallelism of the edges of these cords, without 

 which they will not vibrate in such a manner as to give 

 out sound. 



(c) A certain degree of tightness of the vocal cords, 

 without which they .will not vibrate quickly enough to 

 produce sound. 



(d) The passage of a current of air between the parallel 

 edges of the vocal cords of sufficient power to set the 

 cords vibrating. 



The larynx is a short tubular box opening above into 

 the bottom of the pharynx and below into the top of 

 the trachea. Its framework is supplied by certain carti- 

 lages more or less movable on each other, and these are 

 connected together by joints, membranes, and muscles. 

 Across the middle of the larynx is a transverse partition, 

 formed by two folds of the lining mucous membrane, 

 stretching from either side, but not quite meeting in the 



