ENDOSKELEl'Otf. ANIMALS. 41 



as a resonating chamber connected with hearing, and as a 

 protection to the brain-case The Enstachian tubes by 

 which the tympana open into the pharynx (vide Sect. E) 

 are very complicated, they have three openings on the 

 ventral surface of the skull. Through the unpaired opening 

 blue bristles have been passed into the tympana, and black 

 through the paired. A special passage extends through 

 the inner border of the quadrate bone, and is continued as 

 a membranous tube (siphonium) to an opening in the 

 articular bone of the lower jaw, by this the bone becomes 

 pneumatic. A red bristle has been passed through each of 

 these passages, but the siphonium is not preserved. On 

 the left side the columella auris and part of the membrana 

 tympani is shown. 



van Beneden, Arch, de BioL, t. iii. 1882, p. 497. 



A. 190. Skull of a Rook (Trypanocot'ax frugilegus). The outer 

 wall of the skull has been removed on the left side, exposing 

 a cavity that was occupied by medulla. A portion of the 

 inner wall of this cavity has been removed to show the 

 air-containing chamber. The articular bone of the lower 

 jaw has had its air-chambers exposed, the air being received 

 by means of a bony tube (siphonium) that opens into the 

 tympanic cavity ; a bristle has been passed through the 

 siphonium. 



A. 191. A longitudinal section of two anterior dorsal vertebrae 

 of an Ostrich (Strutkio camelus\ showing the delicate 

 reticular structure of the entire substance of these bones for 

 receiving air, which enters by apertures near the roots of 

 the transverse processes. 0. C. 214. Ilunterian. 



A. 192. The os humeri of an Owl, which shows the cavity free 

 from marrow, and cancellated only at the extremities, and 

 therefore a good receptacle for air. Near to the joint of 

 the shoulder may be observed the aperture by which the 

 air passes into the cavity. 0. C. 210. 



" In most birds, I believe in all that fly, those axillary cells 

 communicate with the cavity of the os humeri by means of small 

 openings in the hollow surface near the head of that bone : in 



