THE SYRINX OF GALLUS DOMESTICUS 197 



that such organs must be capable of exhaling ah' in greater or 

 less amount at the will of the bird. Such structures are found in 

 the air sacs which are present throughout the thoracic and ab- 

 dominal regions and are continuous with the cavities of neigh- 

 boring bones. 



A chicken was anaesthetized, the humerus sawed through at 

 its middle and a tightly fitting piece of rubber tubing placed 

 over the central stump. When air was forced through the tube 

 into the air sacs, by way of the humerus, sound was produced. 

 This varied in pitch with the amount of air pressure used, greater 

 pressure resulting in higher pitch. The trachea was then divided, 

 l)ut this produced no noticeable result, since the bird was unable 

 to control the length of the trachea or the width of the glottis by 

 muscular contraction. Finally, the cer\dcal air sacs were punc- 

 tured after which it was impossible to produce voice in this arti- 

 ficial manner, thus proving Herissaut's statement that when 

 these sacs were ruptured birds are unable to sing. He explains 

 this inabihty to sing as follows: Since the air sacs form air cavi- 

 ties around the syrinx, it can be seen that there is a tendency to 

 equalize pressure on each side of the tympanic membranes, but 

 when sudden gusts of air are forced out through the bronchi the 

 equilibrium is disturbed. Thus the membranes are set into 

 vibration. It is evident that all structm'es through which the 

 voice passes from the syrinx to the exterior act as resonators. 

 So far as size and extent are concerned, the trachea forms a large 

 part of this resonating system. 



Experiment III. In that part of this paper dealing with struc- 

 ture, it was noted that the sterno-trachealis muscles extend along 

 the sides of the trachea from the twelfth ring to the upper larynx. 

 When a pair of electrodes were applied to this muscle in an anaes- 

 thetized chicken, it was seen by its contraction to shorten the 

 trachea from one-fourth to one-third of its original length. It 

 should be recalled that the typical tracheal rings are so arranged 

 that they may overlap when the muscles contract. The trachea 

 may thus act as a pipe or horn capable of being lengthened and 

 shortened. This has a direct influence upon pitch as was shown 

 by the following experiment : The so-called thoracic cavity was 



