MIDDLE-EAR REGION OF GALLUS 255 



While Keith notes the dehiscence in the otic capsule in the 

 amphibians, as established by H. S. Harrison, he does not empha- 

 size the fact that the membrana tympani secundaria in this 

 form is applied to the base of the skull, outside of the territory 

 of the tuba auditiva or that the middle-ear region of the anurans 

 is distinctly respiratory in function, dilating with the injection 

 of air into the lungs. While Keith suggests that we regard the 

 tegmentum vasculosum in birds and the stria vascularis in 

 mammals as merely contributing to the fluid contents of the 

 space, the writer feels they afford a most delicate adjustment to 

 slight variations in pressure, owing to the fact the pressure 

 within the labyrinth and the venous pressure is practically equal. 

 Certainly, this regulation is more delicate than the bending of 

 the membrana tympani secundaria, even granting this force to 

 be less than the friction head against a mass displacement 

 through the helicotrema. 



It has been mentioned that a plane of stability is formed by 

 the suprastapedial and infrastapedial processes and the colu- 

 mellar-squamosal ligament which attach on a line medial to the 

 drum margin. This plane is responsible, in part, for the sup- 

 port to the columella proper. Breuer's experiments on the 

 cochlea have demonstrated that when the cochlear wall is per- 

 forated, the perilymph tends to ooze out in the form of surface 

 droplets and these droplets disappear on a stimulation o': the 

 M. tensor tympani through the N. faciahs. The interpretation, 

 however, that the resulting contraction of the muscle is designed 

 to reduce intralabyrinthine pressure is, however, open to objec- 

 tion. The perilymphatic fluid undoubtedly has a pressure, as 

 has been stated, which is about that of the capillary blood- 

 vessels of this region, and the moment a hole is made in the 

 bony canal, the fluid will naturally ooze out in the form of drop- 

 lets, due also in part to the tendency of the columellar appa- 

 ratus to be displaced inward on loss of tone in the M. tensor 

 tympani. In fixed specimens the membrana tympani secundaria 

 normally appears to bulge slightly into the cavum tympani, and 

 is undoubtedly an artifact. The function of the muscle can be 

 more properly associated with a push factor which tends to 



