MIDDLE-EAR REGION OF GALLUS 239 



process of the quadratum, which is directed upward in front of 

 the drum membrane to articulate with the squamosum, are 

 mainly a fore-and-back hinge motion and very limited near the 

 joint. It must be remembered that the quadratum is inter- 

 mediate in position between the mandible and the squamosum, 

 and because of its attachments through the zygomatic also 

 participates in movement of the upper bill. However, the bone 

 is limited in motion through attachment to the pterygoideum, 

 and in birds with a freely movable upper bill, as in the parrot, 

 the quadratosquamosal articulation is even more pronouncedly 

 a hinge than in birds with little upper bill movement, as in the 

 chicken. A stout ligament usually bridges the tympanic proc- 

 ess of the quadratum at the drum margin. In any event when 

 the quadratum is freed below, it may be violently wrenched out 

 of its socket without any visible influence on drum tension or 

 columellar position, even when the performance is observed 

 under binocular magnifier. The drum attachments are there- 

 fore probably as stable as if an annulus were present, and the 

 movement of the quadratum in reference to the tympanum may 

 be quite disregarded, as far as the chicken is concerned. 



The drum margin is thickened by intrinsic elastic fibers, and 

 in the ventral attachment area, includes a rudimentary air 

 sinus which was at first mistaken for a vein. It is possibly 

 a rudiment of a siphonium canal which connects the air sinuses 

 of the mandible, when they are present, with the cavity of the 

 middle ear just inferior to the foramen pneumaticum for the 

 quadratum and pterygoideum. The marginal sinus, indicated 

 in the semischematic figure 4, in so far as the writer is aware, 

 has not been described, and the result of its position in relation 

 to insulating the drum margin from movement of the quadra- 

 tum has already been mentioned. 



The margin of the drum is thickened postero-inferiorly, where 

 the tendinous fibers of the ]\I. tensor tympani pass to their 

 attachment to the extra- and infrastapedial cartilages. Addi- 

 tional elastic elements are related to the tendinous portion of 

 this muscle, and the drum margin and membrana tympani 

 secundaria appear to be more or less continuous at this point. 



