236 A. G. POHLMAN 



This fibrous plate, bent to conform to the outhne of the canal, 

 afTords origin to the heavy M. digastricus. The lateral portion 

 of the canal is freely movable, so that the bird may shear off 

 communication from the surface to the drum at the point of the 

 kinking. The skin of the canal is loosely applied, particularly 

 over the fibrous plate which attaches the M. digastricus, where a 

 bursa-like slit occurs which continues to the region of the drum. 

 Traction upon the skin of the external canal does not affect the 

 drum membrane, and it may be assumed that the contraction 

 of the M. digastricus or the closing of the external auditory 

 canal in no way influences the middle ear or its contents. 



The drum membrane occupies the upper wall of the canal and 

 looks downward and somewhat backward. It is well hidden 

 from inspection by the kinking in the canal and the convexity 

 of the fibrous plate. This is further accentuated by the presence 

 of the erectile auditory pad • — a semicircular worm-like eleva- 

 tion following the dorsal drum margin and located about 2 mm. 

 lateral to it. The erectile pad is firmly attached at the upper 

 and lower bony borders of the external canal, but its semicircular 

 base is separated from the canal l^y the bursa-like slit. In sec- 

 tion this pad (fig. 1) is a rugous structure, largely composed of 

 connective-tissue elements and some venous spaces. It is 

 limited at its base by stout elastic fibers which are attached to 

 the periosteum near the drum margin, but are not continuous 

 with the drum membrane (fig. 2). The structure was first 

 described by Wurm ('85) as responsible for the deafness in 

 Tetroa urogallus during the period of sexual excitement in that 

 it plugs the external auditory canal through turgescence. This 

 was substantiated by von Graaf ('85), who, however, questions 

 the additional contributing factor suggested by Wurm in a pres- 

 sure exerted by the processus angularis mandibulae against the 

 external canal. The pad itself partakes of the age and sex 

 differences displayed in the comb and wattles, and is larger in 

 the adult than the young bird, and better developed in males 

 than in females. The bursa-like slit separates the pad from the 

 tendinous fibers of the M. tensor tympani which crosses its base 

 at right angles near the drum margin. 



