VOICE AND SPEECH. 421 



ing causes an up-and-down and a lateral sway with each step. Were the legs 

 without joints, like stilts, these oscillations would be very great, especially 

 when the step was long; as a matter of fact, they are slight. The tendency 

 for the centre of gravity to move from side to side as the Legs alternately 

 push the body forward is partly balanced by the swing of the opposite arm ; 

 and the vertical oscillations are minimized by the fact that the leg which is 

 about to receive the weight flexes as the centre of gravity moves forward and 

 comes over it, and extends as it passes on to be received by the other leg. 

 The path taken by the centre of gravity during walking is a complicated one. 

 If referred to the plane in which the body is moving, it describes for one 

 double step an oval ; projected on the horizontal and frontal planes, its path 

 has the form of the sign of infinity, oc. The rate of movement influences its 

 position in special parts of the curve. 1 



In running, the body is inclined more than in walking, and the legs are 

 more flexed in order that the extension movement of the back leg, which 

 drives the body forward, may be more effective. In running, the body is pro- 

 pelled by a series of spring-like movements and there are times when both 

 feet are off the ground, the back leg leaving the ground before the other 

 touches it. 



B. Voice and Speech. 

 1. Structure of the Larynx. 



Voice-production. — The human voice is produced by vibration of the 

 true vocal cords, normally brought about by an expiratory blast of air passing 

 between them while they are approximated and held in a state of tension by 

 muscular action. Mere vibration of the cords could produce but a feeble 

 sound; the voice owes its intensity both to the energy of the expiratory blast 

 (Helmholtz) 2 and to the reinforcement of the vibrations by the resonating 

 cavities above and below the cords. 



A true conception of the action of the larynx can only be gained by a pre- 

 liminary study of the organ in situ, in its relations with the trachea, pharynx, 

 tongue, extrinsic muscles, and hyoidean apparatus. Removed from its con- 

 nections, the larynx, in vertical transverse section, is seen to be shaped some- 

 what like an hour-glass, the true vocal cords forming the line of constriction 

 half way between the top of the epiglottis and the lower border of the cri- 

 coid cartilage (Fig. 208). In median vertical section the axis of the larynx 

 above the vocal cords extends decidedly backward, and below the cords the 

 axis is nearly perpendicular to the plane in which they lie. The epiglottis is an 

 ovoid lamella of elastic cartilage, shaped like a shoe-horn, that leans backward 

 over the laryngeal orifice so that the observer must look down obliquely in 

 order to inspect the cavity of the larynx (Fig. 212.) The mucous membrane 

 is thickened into a slight prominence, known as the "cushion," at the base of 



'Fischer: Abhandl, </. math.-physik. ('I. </. Sachs. QeseUsch. <l. Wissensch., xxv. Nr. i. 

 -Quoted by Griitzner: Hermann's Handi. der Physiologic, L879, Bd. 11. 'I'll. 2, S. 14. 



