VOICE AND SPEECH. 861 



a passive act. Speed in walking is attained by inclining the body somewhat 

 more and by flexing the legs somewhat more, so that the hind limb in extend- 

 ing can push the body forward with greater force. The more rapid move- 

 ment of the body is also accompanied by a more rapid forward swing of the 

 leg, the muscles aiding the force of gravity. The transfer of the weight of 

 the body from one leg to the other causes it to oscillate slightly from side to 

 side, and the falling motion, interrupted by the support offered by the receiving 

 limbs, causes a slight up-and-down movement. These oscillations are, how- 

 ever, very slight ; the tendency for the centre of gravity to move from side to 

 side as the legs alternately push the body forward is in part balanced by the 

 swing of the opposite arm ; and the vertical oscillation is largely obviated, 

 because the supporting leg is extending i. e. lengthening as the body moves 

 forward, and so sustains the pelvis until its weight is taken by the other leg. 



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. The increase in speed is due in part to the greater forward incli- 

 nation of the body, but more especially to the vigorous action of the muscles. 



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) 1 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. 295). In median vertical section the axfs 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. 299). The mucous membrane 

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

 1 Quoted by Griitzner: Hermann's Handb. der Physiologic, Bd. 11, Th. 2, S. 14, 1879. 



