PHYSIOLOGY OF MOVEMENT. 



757 



animal is never employed, but there is always a certain amount of loss 

 from friction and other causes. As most of the contrivances which are 

 employed to enable horses to draw weight consist of attachments which 

 applies the weight to the horse's shoulders, the weight falls in front of the 

 centre of gravity of the body, and the animal may thus be regarded as 

 pushing the weight ; and as the mass moved is the animal's body plus the 

 applied weight, the greater the latter the more the centre of gravity of the 

 common mass will be advanced. Hence, in drawing heavy weights the 

 fore limbs will be behind the common centre of gravity, and they, also, 

 in their extension will aid in propelling the body. 



6. Special Muscular Mechanisms. — The Voice. — By the term voice 

 is meant the sound produced in man and the higher animals through the 

 vibration of the column of air forced by the contraction of the thorax 

 between the vibrating vocal cords of the larynx. Speech, of which man 



FIG. 313.-THE HUMAN LARYNX, AS SEEN ^f^lTl^T^^m^ wJSf 



with the Laryngoscope. (Landois.) ?Landois^ UTTEBING A ±1IGH »otjs. 



L., tongue \ E., epiglottis : F., valleculla; R., glottis; 

 L. v., true vocal corda ; S. M„ sinus Morgagni ; L. y. s„ 

 false vocal cords ; P., position of pharynx ; S., cartilage 

 of Santorini ; W., cartilage of Wrisberg ; S. p., sinus 

 pyriformes. 



alone is capable, consists in certain modifications of the vocal sounds 

 by the parts situated above the larynx; that is, the pharynx, mouth, soft 

 palate, nasal fossae, tongue, teeth, and lips. Speech may, therefore, be 

 described as articulate voice. 



Voice is produced by the imparting of the vibrations of the vocal cords 

 to the column of air within the respiratory organs. The means by which 

 this is accomplished is entirely analogous to that by which sound is 

 produced in reed instruments. The vocal cords consist of free rims of 

 highly elastic membrane whose tension may be varied by muscular action 

 and whose edges may be approximated or separated (Figs. 313 and 314). 

 When the edges of the vocal cords are in close contact, through a strong 

 muscular expiratory motion the air below the vocal cords becomes 

 greatly condensed and finally its tension is sufficient to overcome the 

 resistance of the closed vocal cords; when the vocal cords are thus 



