MIDDLE-EAR REGION OF GALLUS 257 



ear is probably as stable in the matter of fixed boundaries as is 

 that of the mammals. Both groups have an arrangement in the 

 sound-transmitting apparatus, which decreases the amount of 

 drum excursion in the propagation of this motion to the peri- 

 lymphatic space. The two joints in the mammalian ossicular 

 chain are therefore replaced by a series of intrinsic columellar 

 movements which have been spoken of as hinges. The lever 

 action in the ear bones of the mammal finds a functional ana- 

 logue in the columellar system in birds, and in neither instance 

 is this movement probably associated with an attempt to trans- 

 form the character of the sound wave. Recent evidence seems 

 to point to the sound wave transmission as a molecular quantity. 



While there can be little doubt that some changes in acuteness 

 in hearing result from the contractions of the M. tensor tympani 

 and M. stapedius, it is hardly fair to assume at this time that 

 this is their prime function. A similar result is obtained in the 

 eyes. Narrowing the palpebral fissure is accompanied by con- 

 traction of the pupil and therefore increase in sharpness of vision. 

 This is, however, the normal sleep reflex and can scarcely be 

 classed as a function of the lids. 



If the bird and the mammal both enjoy a middle-ear region 

 of relatively stable nature, there is at least one major factor 

 which calls for muscular adjustment, and that is the variable 

 topography of the drum in reference to the inner ear. It might 

 therefore be well to review briefly some of the theories of the 

 muscle function in the mammal. Wales ('0-9) has suggested the 

 possibility of a plus pressure phase in the air content of the 

 middle ear, which he holds is due to a forcible injection, caused 

 by the method of tubal closure. This result is, however, the 

 same as Mangold's ('13) observation in voluntary relaxation of 

 the M. tensor tympani, apparently not accompanied by palatal 

 motion, although, as has been suggested, it might be that the 

 M. tensor veli palati might operate independently from the M. 

 levator palati in these cases. In any event, the drum membrane 

 undergoes a lateral displacement on relaxation of the M. tensor 

 tympani, which has not been satisfactorily explained. The M. 

 stapedius, according to Wales, would prevent a pressure of the 



