758 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



separated air passes between them, and, consequently, rarefaction of the 

 air below takes place, while the cords being elastic, their tension serves 

 to again readily overcome the propulsion of the air from the contraction 

 of the thorax. As a consequence, the edges of the vocal cords are set 

 into rapid vibration and these vibrations are communicated to the column 

 of air both below and above the vocal cords, and as a result a sound 

 which is due to these vibrations is produced ; the vibrations of condensa- 

 tion and rarefaction of the air are the principal causes of the tone, while 

 the cavities above the vocal cords fulfill the part of resonators. 



Sounds produced in this way, like other musical sounds, may vary 

 in regard to their pitch, intensity, and quality. The pitch of the sound 

 will depend upon the length of the vocal cords, the shorter the vocal 

 cords, the more rapid are the vibrations and the higher the pitch of the 

 note produced. The pitch of the note is further dependent upon the 

 degree of tension of the vocal membrane. As in the case of musical 

 instruments of a similar nature, stringed instruments, or reed instru- 

 ments, the pitch of the note is proportionate to the square root of the 

 tension. In man and other mammals in whom vocal cords are present, 

 the pitch of the note may, therefore, be modified by varying degrees of 

 tension of the vocal cords through the action of different muscles. 



The intensity of the sound depends primarily upon the strength of 

 the blast of air, so that, therefore, the more vigorous the expiration, the 

 greater will be the amplitude of the vibration of the vocal cords and the 

 greater will be the intensity of the sound, while the action of the dif- 

 ferent resonators of the vocal organs, through the sympathetic vibrations 

 induced in their cavities by the vibration of the column of air set into 

 motion by the swaying vocal cords, is added to the fundamental tone 

 that is produced and its intensity is modified. 



The timbre, or quality of the vocal sounds, as in other musical in- 

 struments, depends upon the over-tones or harmonics which accompany 

 the fundamental note. Changes in the shape of the different resonating 

 cavities of the vocal organs will, by modifying the prominence of dif- 

 ferent over-tones, account for the difference in the voice of different 

 animals. 



The mode of production and the character of the voice differ very 

 greatly in different members of the animal kingdom. Voice may be pro- 

 duced in all vertebrates possessed of lungs and la^nges, while in fishes, 

 where the respiration is branchial and not pulmonary, the production of 

 voice is impossible. In invertebrates the sounds produced in such great 

 variet}^ are in no respect analogous to the voice, since they are produced 

 by entirety different mechanisms. Thus, insects produce sounds (which 

 are especially distinguished for their acuteness) either by the rapid 

 movement of their wings, as in flies and bees, or by rubbing their legs 



